DISTINCT ISOFORMS OF ADPGLUCOSE PYROPHOSPHORYLASE OCCUR INSIDE AND OUTSIDE THE AMYLOPLASTS IN BARLEY ENDOSPERM

This paper addresses the controversial idea that ADPglucose pyrophosph orylase may be located in the cytosol in some non-photosynthetic plant organs. The intracellular location of the enzyme in developing barley endosperm has been investigated by isolation of intact amyloplasts. A myloplast preparations contained 13-17% of the total endosperm activit y of two plastidial marker enzymes, and less than 0.5% of the total en dosperm activity of two cytosolic marker enzymes. Amyloplast preparati ons contained about 2.5% of the ADPglucose pyrophosphorylase activity, indicating that approximately 15% of the ADPglucose pyrophosphorylase activity in young endosperms is plastidial. Immunoblotting of gels of endosperm and amyloplast extracts also indicated that the enzyme is b oth inside and outside the amyloplast. Antibodies to the small subunit s of the enzyme from barley and maize revealed two bands of protein of different sizes, one of which was located inside and the other outsid e the amyloplast. The plastidial protein was of the same size as a pro tein in the chloroplasts of barley leaves which was also recognized by these antibodies. It is suggested that the barley plant contains two distinct isoforms of ADPglucose pyrophosphorylase: one located in plas tids (chloroplasts and amyloplasts) and the other in the cytosol of th e endosperm. The role of the cytosolic ADPglucose pyrophosphorylase is unknown. Although it may contribute ADPglucose to starch synthesis, t he total activity of ADPglucose pyrophosphorylase in the endosperm is far in excess of the rate of starch synthesis and the plastidial isofo rm is probably capable of catalysing the entire flux of carbon to star ch.