I. theor. Biol. (1977) 69, 287-291
Cometabolism: A Critique MATTHEW H. HULBERT
Department of Chemistry, Connecticut College, New London, CT 06320, U.S.A. AND STEVEN KRAWIEC
Department of Biology, Lehigh University, Bethlehem, PA 18015, U.S.A. (Received 14 June 1976, and in revisedform
23 Ma,v 1977)
Examination of the descriptive features of cometabolism shows that they do not distinguish an activity distinct from ordinary metabolism. Thus, further use of the term “cometabolism” is without merit.
1. A Critique The particulars of intermediary metabolism are impressive in their detail and often seemingly overwhelming in their extent. Generalizations about metabolism based on these particulars and formulated by inductive reasoning are conveniences of great utility. For example, Foster (1888) introduced the general term metabolism to describe biological transformations of substrates into products. He further defined anabolism as constructive metabolism and catabolism as destructive metabolism (Foster, 1888). In 1970, the term “cometabolism” appeared in the microbiological literature to describe the utilization of a particular class of substrates, viz., “cometabolites” (Horvath Kc Alexander, 1970). A survey of the literature has not revealed a precise definition of “cometabolism” (Horvath, 1972). In its absence, we have assembled from Horvath’s review four features commonly associated with “‘cometabolism”. These characteristics are that (i) the “cometabolite” does not support the growth of chemoheterotrophs, (ii) production of waste products is stoichiometrically related to the disappearance of the “cometabolite”, (iii) utilization of the “cometabolite” is associated with increased I ‘) T.B. 287
288
M.
H.
HULRERT
AND
S. KRAWIEC
oxygen consumption, and (iv) “cometabolite” transformation involves adventitious utilization of existing enzyme systems. We assert that not all of these attributes have been established and that none of the remaining attributes individually or collectively is different from metabolism as ordinarily defined. The term “cometabolism”, in our estimation, fails to describe a biologically-meaningful, previously-unrecognized, distinct, and general condition. Rather, we believe that the term arises from the descriptions of the biological consumption of unexpected substrates and that the intrique of “cometabolism” originates with such striking examples of degradative activity as the conversion of DDT to carbon dioxide, chloride, and water (Focht, 1971; Focht & Alexander, 1970; Horvath & Alexander, 1970; Pfaender & Alexander, 1972, 1973; Subba-Rao & Alexander, 1976). The distinctive feature of “cometabolism” in the examples published (Horvath, 1972) appears to be that its observers were surprised by the degradative potential of microbes. Evaluation of the characteristic features associated with “cometabolism” allows an assessment of the validity of the concept. Adequate understanding of the first two characteristics requires careful attention to word usage. Some investigators have observed an absence of replication during the utilization of a “cometabolite” but did not attempt to measure increase in biomass (Focht & Alexander, 1970; Horvath, 1970, 1972; Horvath & Alexander, 1970; Pfaender & Alexander, 1973). Because utilization of the “cometabolite” alone does not result in microbial replication, these authors have stated that “cometabolites” do not support growth. This conclusion is too broad. Further, these authors have claimed that “cometabolites” are neither anabolized nor catabolized. These conclusions do not appear to have been tested; rather they are merely unsupported inferences. The failure of organisms to replicate with only the “cometabolite” as substrate ought not to be surprising since numerous organisms (including wild-type and auxotrophs) require one or more of a wide variety of essential nutrients for replication (Lamanna, Mallette & Zimmerman, 1973). Absence of an essential nutrient will stop replication but not terminate all metabolic activities. Hence, a substrate may be anabolized or catabolized without replication. The second feature - near stoichiometric production of waste products -indicates the occurrence of catabolism rather than establishes the alternative of “cometabolism” (Foster, 1888). Whether released energy is conserved in a biologically useful form (as often but not necessarily occurs in catabolism) has not been reported. The third feature is also not unusual; for example, altered consumptions of molecular oxygen by organisms respiring below their aerobic capacities occur when some supplemental electron donors are available to electron transport systems, when mono-oxygenase
COMETAROLISM
:
A
CRITIQUE
289
activity is uncoupled (cf. Hayaishi, 1974), and, more generally, when the organisms are exposed to stressful conditions (cf. Lamanna et al., 1973; Maizner & HempIling, 1974, 1976). The fourth feature -a happenstance utilization of an enzyme-is, at present, speculation untested by experiment. Although enzyme systems are characterized, in part, by specificity, this specificity has some latitude (Lehninger, 1975). “Cometabolism” may be a fortuitous expression of an enzyme’s range and purely incidental to more prominent enzymatic activities. Another interpretation is that the capacity to transform novel substrates is a preuduptation (Salthe, 1972) which can be selected. The emergence of such a trait can be tested by a long-range chemostat experiment (Tempest, 1970) during which the binding constant for the new substrate is monitored. With one possible exception discussed below, the “cometabolism” hypothesis predicts no benefit to the organism through anabolism or energy-yielding catabolism, i.e. no selective advantage and, thus, would predict no change in the binding constant with time. The more conventional explanation of the catalytic activity, i.e. metabolism with a selective advantage, predicts a p:rogressively lower binding constant (Lehninger, 1975). The one instance in which “cometabolism” may provide a selective advantage is when the “cometabolite” is toxic and the product of “cometabolism” is less so. But such transformations conform to the original definition of catabolism (Foster, 1888); indeed, numerable examples of constitutive and inducible metabolic detoxification have been reported (cf. Connamacher, 1976). The originators of the term “cometabolism” have used it to encompass those conditions that previously had been described as “co-oxidation”; yet “co-oxidation” bears a more exacting definition which is incompatible with the usage of “cometabolism”. “Co-oxidation” refers to the simultaneous oxidation of an organic “co-substrate” which is neither essential for nor sufficient to support the replication of a micro-organism. Psuedomonas methunicu, a methane-dependent organism, incorporates isotopic carbon into cellular material in small amounts during the “co-oxidation” of labelled ethane (Foster, 1962; Leadbetter & Foster, 1960). This example indicates that anabolism of “co-substrates” can occur. In some instances, utilization of “co-substrates” is associated with increased consumption of molecular oxygen. This condition is consistent with catabolic transfer of electrons from the “co-substrate” to molecular oxygen. A further distinction between “cometabolism” and “co-oxidation” is that “co-oxidation” requires the simultaneous presence of a substrate and the “co-substrate”; by contrast, instances of degradation of “cometabolites” in the absence of another substrate have been reported (Horvath, 1972; Horvath & Flathman, 1976). the term “non-growth” clearly Finally, with reference to “co-oxidation”
290
M.
H.
HULBERT
AND
S.
KRAWIEC
means absence of replication; with reference to “cometabolism” the term “non-growth” is vague in meaning. While the definition of “co-oxidation” does not suffer the imprecision and inadequacies of that of “cometabolism”, the concept is not without deficiences. The consumption of “co-substrates” occurs by conventional metabolic processes; hence, the utilization of the “co-substrate” ought not be distinguished by giving the already recognized processes a new name, viz., “co-oxidation”. The differentiating features of “co-oxidation” are in the nature of the reactant: the “co-substrate” alone is not essential and is not sufficient to support replication. Recognition of such a class of reactants reveals no new metabolic phenomenon; rather this recognition contributes to the cataloguing of organic compounds. We conclude that none of the demonstrated features of “cometabolism” differs from those of ordinary catabolism or anabolism. Indeed, it would appear that “cometabolism” is not an intrinsic and distinct activity of an organism but is an expression of the bias of an experimenter : the transformation of a material is termed “cometabolism” because the investigator did not expect the material to be metabolized. The principles of simplicity and economy of explanation indicate that only the long-established categories of metabolism should be retained until such time that a truly different type of metabolism be demonstrated. Further, “cometabolism” falsely suggests that a previously unrecognized capacity for the transformation of substrates exists in some cells. Examples of such an expectation may be seen in recent articles which purport “cometabolic” transformations in the laboratory (cf. Golovleva et al., 1974) and in the environment (cf. Hughes & McKenzie, 1975; Pfister, 1974). Continued use of the term may lead to serious misconceptions about the immediate capacity of micro-organisms to rid the environment of noxious materials. On the basis of the assessment presented here, we propose that use of the word “cometabolism” be abandoned. Specificity of enzymes for substrates should continue to be quantified in terms of binding constants.
REFERENCES CONNAMACHER, R. H. (1976). In Acquired Resistance of Micro-organisms to Chemotherapeutic Drags (F. E. Hahn, ed.), pp. S-66. Base], New York: Karger. FOCHT, D. D. (1971). Bacterial. Proc. 71, 16. FOCHT, D. D. & ALEXANDER, M. (1970). Science 170,91. FOSTER, F. (1888). A Textbook of Physiology, pp. 41-43. New York: MacMillan. FOSTER, J. W. (1962). Antonie van Leeuwenhoek; J. Microbial. Serol. 28, 241. GOLOVLEVA, L. A., ROMANOVA, I. B., NEFEDOVA, M. Yu., CHERVIN, I. I., ADANIN, V. M., VINOKUROVA, N. V. & SKRYABIN, G. K. (1974). Dokl, Akad. Nauk. SSSR Ser. Biol. 219, 485.
COMETABOLISM: H'AYAISHI, 0. (1974). pp. l-28.
291
A CRITIQUE
In Molecular Mechanisms of Oxygen New York and London: Academic Press. R. S. (1970). Biochem. J. 119,871. R. S. (1972). Bacterial. Rev. 36, 146.
Activation (0.
Hayaishi,
ed.).
HORVATH, HORVATH, HORVATH, R. S. & ALEXANDER, M. (1970). Can. J. Microbiof. 16, 1131. HORVATH, R. S. & FLATHMAN, P. (1976). Appl. Environ. Microbial. 31, 889. HUGHES. D. E. & MCKENZIE. P. (1975). Proc. R. Sot. Land. B. 189. 375. LAMANNA, C., MALLETTE, M.‘F. & ZI&WN, L. N. (1973). Basic Bacteriology, 4th edn, pp. 701-705. Baltimore: Williams & Wilkins. LZADBE~TER, E. R. & FOSTER, J. W. (1960). Arch. Mikrobiol. 35,92. LEHNINGER, A. L. (1975). Biochemistry, 2nd edn, pp. 189-194,217-218. New York: Worth. h~..uzNErt, S. E. & HEMPFLING, W. P. (1974). Abs. Annu. Meet. Am. Sot. Microbial. 38. MAIZNER, S. E. & HEMPFLING, W. P. (1976). J. Bacterial. 126, 251. PFAENDER. F. K. & ALEXANDER. M. (19721. J. Aaric. Food Chem. 20. 842. FFAENDER; F. K. & ALEXANDER; M. (1973j. J. &ric. Food Chem. 21; 397. FFISTER, R. M. (1974). In Microbial Ecology (A. I. Laskin & H. L&chevalier, eds), pp. l-27. Cleveland : CRC Press. SALTHE, S. M. (1972). Evolutionary Biology. New York: Holt, Rinehart & Winston. SUBBA-RAO, R. V. & ALEXANDER, M. (1976). Abs. Annu. Meet. Am. Sot. MicrobioI., 201. TEMPEST, D. W. (1970). In Advances in Microbial Physiology (A. H. Rose & J. F. Wilkinson, eds), Vol. 4, p. 245. London: Academic Press.
Cometabolism: A Critique MATTHEW H. HULBERT
Department of Chemistry, Connecticut College, New London, CT 06320, U.S.A. AND STEVEN KRAWIEC
Department of Biology, Lehigh University, Bethlehem, PA 18015, U.S.A. (Received 14 June 1976, and in revisedform
23 Ma,v 1977)
Examination of the descriptive features of cometabolism shows that they do not distinguish an activity distinct from ordinary metabolism. Thus, further use of the term “cometabolism” is without merit.
1. A Critique The particulars of intermediary metabolism are impressive in their detail and often seemingly overwhelming in their extent. Generalizations about metabolism based on these particulars and formulated by inductive reasoning are conveniences of great utility. For example, Foster (1888) introduced the general term metabolism to describe biological transformations of substrates into products. He further defined anabolism as constructive metabolism and catabolism as destructive metabolism (Foster, 1888). In 1970, the term “cometabolism” appeared in the microbiological literature to describe the utilization of a particular class of substrates, viz., “cometabolites” (Horvath Kc Alexander, 1970). A survey of the literature has not revealed a precise definition of “cometabolism” (Horvath, 1972). In its absence, we have assembled from Horvath’s review four features commonly associated with “‘cometabolism”. These characteristics are that (i) the “cometabolite” does not support the growth of chemoheterotrophs, (ii) production of waste products is stoichiometrically related to the disappearance of the “cometabolite”, (iii) utilization of the “cometabolite” is associated with increased I ‘) T.B. 287
288
M.
H.
HULRERT
AND
S. KRAWIEC
oxygen consumption, and (iv) “cometabolite” transformation involves adventitious utilization of existing enzyme systems. We assert that not all of these attributes have been established and that none of the remaining attributes individually or collectively is different from metabolism as ordinarily defined. The term “cometabolism”, in our estimation, fails to describe a biologically-meaningful, previously-unrecognized, distinct, and general condition. Rather, we believe that the term arises from the descriptions of the biological consumption of unexpected substrates and that the intrique of “cometabolism” originates with such striking examples of degradative activity as the conversion of DDT to carbon dioxide, chloride, and water (Focht, 1971; Focht & Alexander, 1970; Horvath & Alexander, 1970; Pfaender & Alexander, 1972, 1973; Subba-Rao & Alexander, 1976). The distinctive feature of “cometabolism” in the examples published (Horvath, 1972) appears to be that its observers were surprised by the degradative potential of microbes. Evaluation of the characteristic features associated with “cometabolism” allows an assessment of the validity of the concept. Adequate understanding of the first two characteristics requires careful attention to word usage. Some investigators have observed an absence of replication during the utilization of a “cometabolite” but did not attempt to measure increase in biomass (Focht & Alexander, 1970; Horvath, 1970, 1972; Horvath & Alexander, 1970; Pfaender & Alexander, 1973). Because utilization of the “cometabolite” alone does not result in microbial replication, these authors have stated that “cometabolites” do not support growth. This conclusion is too broad. Further, these authors have claimed that “cometabolites” are neither anabolized nor catabolized. These conclusions do not appear to have been tested; rather they are merely unsupported inferences. The failure of organisms to replicate with only the “cometabolite” as substrate ought not to be surprising since numerous organisms (including wild-type and auxotrophs) require one or more of a wide variety of essential nutrients for replication (Lamanna, Mallette & Zimmerman, 1973). Absence of an essential nutrient will stop replication but not terminate all metabolic activities. Hence, a substrate may be anabolized or catabolized without replication. The second feature - near stoichiometric production of waste products -indicates the occurrence of catabolism rather than establishes the alternative of “cometabolism” (Foster, 1888). Whether released energy is conserved in a biologically useful form (as often but not necessarily occurs in catabolism) has not been reported. The third feature is also not unusual; for example, altered consumptions of molecular oxygen by organisms respiring below their aerobic capacities occur when some supplemental electron donors are available to electron transport systems, when mono-oxygenase
COMETAROLISM
:
A
CRITIQUE
289
activity is uncoupled (cf. Hayaishi, 1974), and, more generally, when the organisms are exposed to stressful conditions (cf. Lamanna et al., 1973; Maizner & HempIling, 1974, 1976). The fourth feature -a happenstance utilization of an enzyme-is, at present, speculation untested by experiment. Although enzyme systems are characterized, in part, by specificity, this specificity has some latitude (Lehninger, 1975). “Cometabolism” may be a fortuitous expression of an enzyme’s range and purely incidental to more prominent enzymatic activities. Another interpretation is that the capacity to transform novel substrates is a preuduptation (Salthe, 1972) which can be selected. The emergence of such a trait can be tested by a long-range chemostat experiment (Tempest, 1970) during which the binding constant for the new substrate is monitored. With one possible exception discussed below, the “cometabolism” hypothesis predicts no benefit to the organism through anabolism or energy-yielding catabolism, i.e. no selective advantage and, thus, would predict no change in the binding constant with time. The more conventional explanation of the catalytic activity, i.e. metabolism with a selective advantage, predicts a p:rogressively lower binding constant (Lehninger, 1975). The one instance in which “cometabolism” may provide a selective advantage is when the “cometabolite” is toxic and the product of “cometabolism” is less so. But such transformations conform to the original definition of catabolism (Foster, 1888); indeed, numerable examples of constitutive and inducible metabolic detoxification have been reported (cf. Connamacher, 1976). The originators of the term “cometabolism” have used it to encompass those conditions that previously had been described as “co-oxidation”; yet “co-oxidation” bears a more exacting definition which is incompatible with the usage of “cometabolism”. “Co-oxidation” refers to the simultaneous oxidation of an organic “co-substrate” which is neither essential for nor sufficient to support the replication of a micro-organism. Psuedomonas methunicu, a methane-dependent organism, incorporates isotopic carbon into cellular material in small amounts during the “co-oxidation” of labelled ethane (Foster, 1962; Leadbetter & Foster, 1960). This example indicates that anabolism of “co-substrates” can occur. In some instances, utilization of “co-substrates” is associated with increased consumption of molecular oxygen. This condition is consistent with catabolic transfer of electrons from the “co-substrate” to molecular oxygen. A further distinction between “cometabolism” and “co-oxidation” is that “co-oxidation” requires the simultaneous presence of a substrate and the “co-substrate”; by contrast, instances of degradation of “cometabolites” in the absence of another substrate have been reported (Horvath, 1972; Horvath & Flathman, 1976). the term “non-growth” clearly Finally, with reference to “co-oxidation”
290
M.
H.
HULBERT
AND
S.
KRAWIEC
means absence of replication; with reference to “cometabolism” the term “non-growth” is vague in meaning. While the definition of “co-oxidation” does not suffer the imprecision and inadequacies of that of “cometabolism”, the concept is not without deficiences. The consumption of “co-substrates” occurs by conventional metabolic processes; hence, the utilization of the “co-substrate” ought not be distinguished by giving the already recognized processes a new name, viz., “co-oxidation”. The differentiating features of “co-oxidation” are in the nature of the reactant: the “co-substrate” alone is not essential and is not sufficient to support replication. Recognition of such a class of reactants reveals no new metabolic phenomenon; rather this recognition contributes to the cataloguing of organic compounds. We conclude that none of the demonstrated features of “cometabolism” differs from those of ordinary catabolism or anabolism. Indeed, it would appear that “cometabolism” is not an intrinsic and distinct activity of an organism but is an expression of the bias of an experimenter : the transformation of a material is termed “cometabolism” because the investigator did not expect the material to be metabolized. The principles of simplicity and economy of explanation indicate that only the long-established categories of metabolism should be retained until such time that a truly different type of metabolism be demonstrated. Further, “cometabolism” falsely suggests that a previously unrecognized capacity for the transformation of substrates exists in some cells. Examples of such an expectation may be seen in recent articles which purport “cometabolic” transformations in the laboratory (cf. Golovleva et al., 1974) and in the environment (cf. Hughes & McKenzie, 1975; Pfister, 1974). Continued use of the term may lead to serious misconceptions about the immediate capacity of micro-organisms to rid the environment of noxious materials. On the basis of the assessment presented here, we propose that use of the word “cometabolism” be abandoned. Specificity of enzymes for substrates should continue to be quantified in terms of binding constants.
REFERENCES CONNAMACHER, R. H. (1976). In Acquired Resistance of Micro-organisms to Chemotherapeutic Drags (F. E. Hahn, ed.), pp. S-66. Base], New York: Karger. FOCHT, D. D. (1971). Bacterial. Proc. 71, 16. FOCHT, D. D. & ALEXANDER, M. (1970). Science 170,91. FOSTER, F. (1888). A Textbook of Physiology, pp. 41-43. New York: MacMillan. FOSTER, J. W. (1962). Antonie van Leeuwenhoek; J. Microbial. Serol. 28, 241. GOLOVLEVA, L. A., ROMANOVA, I. B., NEFEDOVA, M. Yu., CHERVIN, I. I., ADANIN, V. M., VINOKUROVA, N. V. & SKRYABIN, G. K. (1974). Dokl, Akad. Nauk. SSSR Ser. Biol. 219, 485.
COMETABOLISM: H'AYAISHI, 0. (1974). pp. l-28.
291
A CRITIQUE
In Molecular Mechanisms of Oxygen New York and London: Academic Press. R. S. (1970). Biochem. J. 119,871. R. S. (1972). Bacterial. Rev. 36, 146.
Activation (0.
Hayaishi,
ed.).
HORVATH, HORVATH, HORVATH, R. S. & ALEXANDER, M. (1970). Can. J. Microbiof. 16, 1131. HORVATH, R. S. & FLATHMAN, P. (1976). Appl. Environ. Microbial. 31, 889. HUGHES. D. E. & MCKENZIE. P. (1975). Proc. R. Sot. Land. B. 189. 375. LAMANNA, C., MALLETTE, M.‘F. & ZI&WN, L. N. (1973). Basic Bacteriology, 4th edn, pp. 701-705. Baltimore: Williams & Wilkins. LZADBE~TER, E. R. & FOSTER, J. W. (1960). Arch. Mikrobiol. 35,92. LEHNINGER, A. L. (1975). Biochemistry, 2nd edn, pp. 189-194,217-218. New York: Worth. h~..uzNErt, S. E. & HEMPFLING, W. P. (1974). Abs. Annu. Meet. Am. Sot. Microbial. 38. MAIZNER, S. E. & HEMPFLING, W. P. (1976). J. Bacterial. 126, 251. PFAENDER. F. K. & ALEXANDER. M. (19721. J. Aaric. Food Chem. 20. 842. FFAENDER; F. K. & ALEXANDER; M. (1973j. J. &ric. Food Chem. 21; 397. FFISTER, R. M. (1974). In Microbial Ecology (A. I. Laskin & H. L&chevalier, eds), pp. l-27. Cleveland : CRC Press. SALTHE, S. M. (1972). Evolutionary Biology. New York: Holt, Rinehart & Winston. SUBBA-RAO, R. V. & ALEXANDER, M. (1976). Abs. Annu. Meet. Am. Sot. MicrobioI., 201. TEMPEST, D. W. (1970). In Advances in Microbial Physiology (A. H. Rose & J. F. Wilkinson, eds), Vol. 4, p. 245. London: Academic Press.
