D-Lactate dehydrogenases from t h e foot muscles of the black abalone, Haliotis cracherodii, and t h e pulmonate snail, Helix aspersa, have been characterized. The D-lactate dehydrogenase from H. cracherodii, which is found as a single, isoenzymic form, was purified to homogeneity. The enzyme in H. aspersa foot muscle is found as two distinct fast and slow migrating electrophoretic complexes of isoenzymes. The most abundant isoenzyme in t h e slow electrophoretic complex of H. aspersa was purified to homogeneity. The purified enzymes from t h e two organisms have similar physical and catalytic properties to one another and to other D-lactate dehydrogenases, suggesting a close evolutionary relationship. Purified D-lactate dehydrogenases from H. cracherodii and H. aspersa have molecular weights of approximately 80,000, as judged by meniscus depletion sedimentation equilibrium experiments, which correspond to a dimeric subunit organization. Kinetic studies show t h a t t h e apparent Michaelis constant for pyruvate is 0.2-0.3 mM for the two purified enzymes as well as the partially purified fast isoenzymic complex from snail. K, values for D-lactate a r e about ten times higher. K, values for u-ketobutyrate are 4-10 mM for all three enzymes and V, , , is approximately 40-80'% of t h a t seen with pyruvate, suggesting that the enzymes may be physiologically important in dealing with u-ketoacids in general. K,'s for NADH are all 10-20 pM, and those for NAD' about 10-fold greater. The most notable catalytic difference between t h e two enzymes is the presence and absence of substrate inhibition by pyruvate of t h e abalone and snail enzymes, respectively. This difference may reflect different physiological roles for t h e enzyme in t h e two organisms.
We thank the California Fish and Game Commission for perm is^ sion to collect specimens of Haliotis cracherodii.
' D-Lactate concentrations were 4 0 mM for Haliotis and 16.0 mM for Helm enzymes ' Pyruvate concentrations were 0.8 mM for Halrotrs a n d 5.0 mM for Helix enzymes