OPHIDIAN l-AMINO ACID OXIDASE. THE NATURE OF THE ENZYME-SUBSTRATE COMPLEXES

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Abstract
To investigate the kinetics of ophidian L-amino acid oxidase, V and Km were determined for phenylalanines that were substituted in every ring position with groups of various size and reactivity, and for a few ring-substituted tryptophans and histidines. The venom of one representative from each of three major classes of poisonous snakes, Naja melanoleuca, Vipera russelli and Crotalus adamanteus, served as a source of the ophidian L-amino acid oxidase. Both crude and crystalline enzyme from the venom of C, adamanteus were tested. The introduction of a benzene ring into glycine and alanine caused some increase of V and a very marked depression of Km. With the exception of fluorine, residues in the ortho position of phenylalanine led to a decrease of V. The rates induced by various substitutions follow the pattern meta [greater than or equal to] para [greater than or equal to] ortho. Within the halogen series, the effects become more pronounced with increasing atomic number. Ring substitution in heter-ocyclic amino acids also affected the V values markedly. For methyl-substituted tryptophans the pattern was: 5-methyl [greater than or equal to] 6-methyl [greater than or equal to] 4-me-thyl. In a few instances ring substition accounts for a considerable elevation of V, as shown for [beta]-quinol-4-ylalanine and its 6-methoxy derivative. The kinetic constants appear to be unaffected by relatively high concentrations of the corresponding D-amino acids. A general principle that permits a uniform interpretation of a vast body of information is suggested. It is based on the assumption that most substrates form not only eutopic but also dystopic complexes with the enzyme. The latter, in contrast with the former, do not permit the formation of reaction products. K values for eutopic and dystopic complexes are computed. Similar concepts have been presented to elucidate the action of [alpha]-chymotrypsin (Hein & Niemann, 1962) and of monoamine oxidase.