Purification and Properties of Nonproteolytic Degraded ADPglucose Pyrophosphorylase from Maize Endosperm

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Abstract
ADPglucose pyrophosphorylase from developing endosperm tissue of starchy maize (Zea mays) was purified 88-fold to a specific activity of 34 micromoles α-glucose-1-P produced per minute per milligram protein. Rabbit antiserum to purified spinach leaf ADPglucose pyrophosphorylase was able to inhibit pyrophosphorolysis activity of the purified enzyme by up to 90%. The final preparation yielded four major protein staining bands following sodium dodecyl sulfate polyacrylamide gel electrophoresis. When analyzed by Western blot hybridization only the fastest migrating, 54 kilodaltons, protein staining band cross-reacted with affinity purified rabbit antispinach leaf ADPglucose pyrophosphorylase immunoglobulin. The molecular mass of the native enzyme was estimated to be 230 kilodaltons. Thus, maize endosperm ADPglucose pyrophosphorylase appears to be comprised of four subunits. This is in contrast to the respective subunit and native molecular masses of 96 and 400 kilodaltons reported for a preparation of maize endosperm ADPglucose pyrophosphorylase (Fuchs RL and JO Smith 1979 Biochim Biophys Acta 556: 40-48). Proteolytic degradation of maize endosperm ADPglucose pyrophosphorylase appears to occur during incubation of crude extracts at 30°C or during the partial purification of the enzyme according to a previously reported procedure (DB Dickinson, J Preiss 1969 Arch Biochem Biophys 130: 119-128). The progressive appearance of a 53 kilodalton antigenic peptide suggested the loss of a 1 kilodalton proteolytic fragment from the 54 kilodalton subunit. The complete conservation of the 54 kilodalton subunit structure following extraction of the enzyme in the presence of phenylmethylsulfonyl fluoride and/or chymostain was observed. The allosteric and catalytic properties of the partially purified proteolytic degraded versus nondegraded enzyme were compared. The major effect of proteolysis was to enhance enzyme activity in the absence of added activator while greatly decreasing its sensitivity to the allosteric effectors 3-P-glycerate and inorganic phosphate.