Capacity for NADPH/NADP turnover in the cytosol of barley seed endosperm: The role of NADPH-dependent hydroxypyruvate reductase

Barley (Hordeum vulgare L.) endosperm from developing seeds was found to co ntain relatively high activities of cytosolic NAD(P)II-dependent hydroxypyr uvate reductase (HPR-2) and isocitrate dehydrogenase (ICDH). In contrast, a ctivities of peroxisomal NADH-dependent hydroxypyruvate reductase (HPR-1) a nd glycolate oxidase as well as cytosolic NAD(P)H-dependent glyoxylate redu ctase were very low or absent in the endosperm both during maturation and s eed germination, indicating the lack of a complete glycolate cycle in this tissue. In addition, activities of cytosolic glucose-6-phosphate dehydrogen ase and glyceraldehyde-3-phosphate dehydrogenase were low or absent in the endosperm. The endosperm HPR-2 exhibited similar properties to those of an earlier described HPR-2 from green leaves, e.g. activities with both hydrox ypyruvate and glyoxylate, utilization of both NADPH and NADH as cofactors, and a strong uncompetitive inhibition by oxalate (K-i in the order of micro molar). In etiolated leaves, both HPR-1 and HPR-2 were present with the sam e activity as in green leaves, indicating that the lack of HPR-I in the end osperm is not a general feature of non-photosynthetic tissues. We conclude that the endosperm has considerable capacity for cytosolic NADP/NADPH cycli ng via HPR-2 and ICDH, the former being possibly involved in the utilizatio n of a serine-derived carbon. (C) 2000 Editions scientifiques et medicales Elsevier SAS.