The role of coenzymes in dehydrogenase systems

Abstract

2 typical cozymase-dependent dehydrogenases, the alcohol and malic dehydro-genases of yeast, were studied with a wider range of hydrogen acceptors. The H-acceptors fell into 3 groups: (a) those which required the presence of cozymase and flavoprotein; dyes (with certain exceptions), m-dinitrobenzene (probably), cytochrome etc.; (b) those which required the presence of cozymase but not flavoprotein; benzoquinone, o-quinone, phenazine dyes, K3Fe(CN)s; and (c) those which did not require the presence of either cozymase or flavoprotein: alloxan, I2, H2O2, dibromophenol-indophenol. Cozymase was not necessary for all acceptors. Cozymase was only one of several acceptors which reacted directly with the activated substrate. Two independent methods showed the dehydrogenases alone (i.e., the so-called "proteins") brought about the oxidation of their respective substrates in the complete absence of cozymase when acceptors of group (c) are used. Thus the "proteins" were complete enzymes. The relationship of dehydrogenase to coenzyme was that of enzyme to substrate rather than that of protein to prosthetic group. The coenzymes were not regarded as the prosthetic groups of the dehydrogenases, and the conception of "pyridine-proteins" was considered misleading.