Quantitative estimates of the distribution of homoserine dehydrogenase isozymes in maize tissues

The low molecular weight threonine-resistant (class I) and the higher molecular weight threonine-sensitive (class II/III) isozymes of homoserine dehydrogenase (EC 1.1.1.3) isolated from Zea mays L. were shown to differ in stability during incubations in the presence of urea. Class II/III was inactivated by urea in a time- and concentration-dependent manner, with complete inactivation occurring within 24 hours at 5 degrees C in 4.0 m urea. Under identical conditions, neither the activity nor the properties of class I were affected. Therefore, it was possible to estimate the amounts and properties of both maize isozymes in crude mixtures by measurements of enzyme activity before and after treatment with urea.The relative amounts of the two isozymes proved to be tissue-specific. When shoots of etiolated seedlings were extracted under optimum conditions, the resultant preparations contained about 16% class I and 84% class II/III. This distribution of isozymes, as well as the regulatory properties of class II/III, were constant during growth of the seedlings between 4 and 13 days. Enzyme preparations isolated from shoots of light-grown plants contained higher proportions of class I. The two isozymes were not uniformly distributed within leaves, as the basal meristematic region contained high levels of II/III and small amounts of I. During leaf maturation, the amount of II/III declined while the level of I remained constant or increased slightly. As a result, nearly half of the enzyme extracted from leaf tips was class I. The synthesis of specific members of the aspartate family of amino acids might be expected to differ when the ratio of threonine-sensitive to threonine-resistant homoserine dehydrogenase is altered. However, additional information on the subcellular localization and the catalytic characteristics of the two enzymes is required for evaluation of this possibility.