Tissue specific variations in the urea sensitivity of the e(1) esterase in maize.

N a previous publication (SCHWARTZ 1964a) evidence was presented to the I effect that enzymes specified by the same allele functioning in two different parts of the maize plant are not identical. The NN pH 7.5 esterases synthesized under the control of the E: allele in the seedling and endosperm show identical electrophoretic migration rates in starch gel electrophoresis; however, they differ in their inactivation by urea. This finding was interpreted as reflecting a difference in the total structure of the enzymes. The enzymes could vary in the compactness of folding around the active site, their conjugation with uncharged cofactors which could influence enzyme stability, or the like. These results were based on comparison of the relative intensities of the pH 7.5 esterase bands in zymograms developed after electrophoresis of control and urea-treated homogenates of the two plant parts. This is not a very quantitative method for inactivation studies, but the method was employed in order to compare the urea sensitivity of the enzymes in the crude preparations minimizing possible alterations of the structure of the enzyme by purification procedures. Comparison of total esterase activity in treated and untreated crude homogenates is meaningless since both plumule and endosperm material contain a number of esterases specified by other genes. In this paper, we report the results of our studies on the comparison of the NN pH 7.5 esterase of the two plant parts utilizing partially purified enzyme preparations. Since this enzyme preparation is free of all the other esterases specified by nonallelic genes, more quantitative procedures could be employed for assay of enzymatic activity. The results confirm the previously reported tissue specific differences in urea sensitivity of the NN esterase. These differences are eliminated when the enzyme is converted to an acidic form by treatment with glyceraldehyde.