53 catalytic interaction between enzyme-coenzyme complex and substrate (VENNESLAND and WESTHEIMER 1954; VENNESLAND 1955,1956; GUTFREUND 1959) have all been the subject of numerous excellent and exhaustive reviews. These topics, therefore, will not be treated in extenso here. The present summary will deal instead with more recent observations on the pyridine nucleotide dependent dehydrogenases which have necessitated a re-examination of their function and structure in the light of new information concerning their composition. The new compositional characterization of these enzymes dates from 1955 [VALLEE and HOCH 1955 (a), (b), (c)] when the DPN dependent alcohol dehydrogenase of bakers' yeast was identified as a zinc metalloenzyme. Since then several other DPN -dependent enzymes have been similady shown to contain zinc [VALLEE 1955, 1960 (a), (b)], and a number of DPN and TPN dependent dehydrogenases have been tentatively identified as metalloenzymes (VALLEE et al. 1956). Knowledge of the presence of zinc in these enzymes has provided a new set of parameters for the study of these systems. The metal in such systems retains much of the chemical reactivity exhibited in the ionic form, modified by its incorporation into the protein matrix. Since the metal is not only a compositional constituent, but is also involved in the protein structure and in the events of catalysis, its reactivity can be used to advantage in exploring specific enzymatic sites, and the general problems of biological catalysis.