ADP-glucose pyrophosphorylase from wheat endosperm. Purification and characterization of an enzyme with novel regulatory properties.

ADP-glucose pyrophosphorylase (AGPase; EC 2.7.7.27) was purified and characterized from two wheat (Triticum aestivum L.) tissues: leaf and endosperm. The leaf enzyme, purified over 1,300-fold, was found to be a heterotetramer composed of subunits of 51 and 54 kDa and possessing regulatory properties typical of AGPases from photosynthetic tissues, being mainly regulated by 3-phosphoglycerate (activator; A0.5=0.01 mM) and orthophosphate (inhibitor; I0.5=0.2 mM). Conversely, the enzyme from wheat endosperm was insensitive to activation by 3-phosphoglycerate and other metabolites. It was, however, inhibited by orthophosphate (I0.5=0.7 mM), ADP (I0.5=3.2 mM) and fructose-1,6-bisphosphate (0.5 = 1.5 mM). All of these inhibitory actions were reversed by 3-phosphoglycerate and fructose-6-phosphate. The endosperm enzyme was found to be a heterotetramer composed of subunits of 52 and 53 kDa, which were recognized by antiserum raised to spinach leaf AGPase. The results suggest that wheat endosperm AGPase possesses distinctive regulatory properties that are relevant in vivo.