beta-Aspartokinase from developing endosperm of maize.

β-aspartokinase (EC 2.7.2.4.) has been isolated from the developing endosperm (30 days post-pollination) of Zea mays (cv. Pioneer 3145). Enzyme activity was dependent upon ATP, Mg⁺⁺ or Mn⁺⁺, aspartate, and protein concentration. Double reciprocal plots of velocity vs. aspartate concentrations deviated from a straight line at low aspartate concentration indicating two apparent Km's of 0.5 and 6.6 mM. Enzyme activity was inhibited by lysine but not by methionine or threonine. The endosperm-derived β-aspartokinase behaved similarly to enzyme isolated from 6-day-old etiolated shoot tissue. The presence of β-aspartokinase in developing endosperm provides new insight into the source of the aspartate-derived amino acids in maize endosperm.