Plant NAD(H)-Glutamate Dehydrogenase Consists of Two Subunit Polypeptides and Their Participation in the Seven Isoenzymes Occurs in an Ordered Ratio

Abstract

The structure and function of NAD(H)-glutamate dehydrogenase in plants was studied by using grapevine (Vitis vinifera L. cv Sultanina) callus grown under different nitrogen sources. The enzyme consists of two subunit-polypeptides, alpha and beta, with similar antigenic properties but with different molecular mass and charge. The two polypeptides have molecular masses of 43.0 and 42.5 kilodaltons, respectively. The holoenzyme is hexameric and is resolved into seven isoenzymes by native gel electrophoresis. Two-dimensional native/SDS-PAGE revealed that the 1 and 7 isoenzymes are homohexamers and the isoenzymes 2 through 6 are hybrids of the two polypeptides following an ordered ratio. The total quantity of alpha- and beta-polypeptides and the isoenzymic pattern was altered by the exogenous nitrogen source. The sample derived from callus grown on nitrate or glutamic acid contained a slightly greater amount of beta-polypeptide and of the more cathodal isoenzymes, whereas alpha-polypeptide and the more anodal isoenzymes predominated in callus grown in the presence of either ammonium or glutamine. The anabolic reaction was correlated with the alpha- and the catabolic reaction with the beta-polypeptide; this could suggest that each isoenzyme exhibits anabolic and catabolic function of different magnitude. The isoenzymic patterns did not obey the expected binomial distribution proportions.