Changes in Enzyme Regulation during Growth of Maize: II. Relationships among Multiple Molecular Forms of Homoserine Dehydrogenase.

The relative contribution of each of several forms of homoserine dehydrogenase (EC 1.1.1.3) to the total enzyme population in etiolated shoots and in roots of Zea mays L. var. earliking was examined by the use of gel filtration chromatography and disc gel electrophoresis. In enzyme preparations derived from shoots of seedlings grown for 72, 120, or 168 hours, two molecular forms, II and III, which have the same apparent molecular weight but differ in net charge, contributed 75 to 80% of the total enzyme activity. A lower molecular weight species, form I, contributed 20 to 25% of the activity from 72-hour shoots, but was found to decrease concomitantly with a proportional increase in activity contributed by aggregated enzyme form(s) during shoot development. Form I contributed a comparatively larger fraction of the total enzyme activity in preparations of roots of 72-hour seedlings.The characteristic enzyme activity of different tissues was found to be the result of variations in both the amount and the properties of individual forms. Form I was consistently insensitive to inhibition by the feed-back modifier, l-threonine, but evidence is presented which indicates that the regulatory properties of form II and/or form III are systematically altered during shoot growth. The activity of the enzyme forms was also differentially stimulated by monovalent cations, K(+) being the most effective activator; in all cases the potential for activation was correlated with the potential for inhibition. In contrast to these differences among the forms of the maize enzyme, all forms were shown to share a number of common characteristics. Potential factors which could influence the growth-associated changes in homoserine dehydrogenase are discussed briefly.