Essential arginine residues in beef kidney D-aspartate oxidase (a preliminary report)

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

Chemical modification of functional arginyl residues in beef kidney D-aspartate oxidase

Chemical modification of beef kidney D-aspartate oxidase by phenylglyoxal is a biphasic process involving the transient formation of an enzymatic species with a decreased activity versus dicarboxylic substrates, an increased activity versus D-proline and a new activity versus other monocarboxylic D-amino acids which is absent in the native protein. Prolonged incubation with the modifier causes complete inactivation of the enzyme. The presence of the competitive inhibitor L-tartrate in the incubation mixture prevents enzyme inactivation. Kinetic and structural data suggest that complete loss of activity is paralleled by modification of eight arginine residues, of which two are critical for the specificity and the activity of the enzyme. We propose that the two essential arginine residues are located in the substrate binding site of D-aspartate oxidase.

Inactivation of L-lactate monooxygenase with 2,3-butanedione and phenylglyoxal

L-Lactate monooxygenase from Mycobacterium phlei is inactivated by reaction either with 2,3-butanedione in borate or in 2,6-lutidine buffer or with phenyglyoxal in 2,6-lutidine buffer. The activation with 2.3 butanedione in borate buffer is irreversible in the presence of excess borate, but essentially complete recovery of activity occurs on exchange of phosphate for borate buffer. In 50 mM borate, inactivation with 2,3-butanedione exhibits saturation kinetics with respect to increasing concentrations of 2,3-butanedione, whereas second-order kinetics for inactivation are seen in 200 mM borate. In 2.6-lutidine buffer, the inactivation is rapid, irreversible on change of buffer, and second order overall. Complete inactivation of the enzyme by phenylglyoxal in 2,6-lutidine buffer occurs on incorporation of 2 equiv of phenylglyoxal per subunit, but only one arginyl residue per subunit is modified. The inactivation is irreversible and second order in phenyglyoxal. There is substantial protection from inactivation in the presence of D-lactate, a competitive inhibitor of the enzyme. It is suggested that an arginyl residue in the active site in L-lactate monooxygenase is involved in the binding of the carboxyl group of substrates to the enzyme. An explanation for the unusual kinetics of inactivation with 2,3-butanedione in borate and with phenylglyoxal in 2,6-lutidine is offered.

[Function of arginine in enzymes]

The average arginine content of proteins is 3.9%. Its frequency among the 20 amino acids of the proteins (13th position) is far lower than would be anticipated from the fact that 6 of the 61 codons for amino acids in the genetic code are arginine codons. Possible explanations for the relatively low frequency of arginine in proteins are discussed. The chemical and physicochemical properties of arginine, which are determined by the guanido group, and the method for chemical modification of arginine residues in proteins are described. The most important function of arginine residues in enzymes seems to be the recognition, binding, and orientation of anionic substrates and cofactors. This function is illustrated by numerous examples.