NAD(P)H-UTILIZING OXIDOREDUCTASES OF THE PLASMA-MEMBRANE - AN OVERVIEW OF PRESENTLY PURIFIED PROTEINS

A considerable number of studies have demonstrated the presence of NAD(P)-oxidoreductases in the plant and animal cell plasma membranes. Recently several attempts on the isolation and purification of these proteins have been presented. The results indicate the presence of distinct NAD(P)H-utilizing enzymes in the plasma membrane of several species. Proteins with molecular masses of 27 kDa, 31 kDa, 36-39 kDa, and 45 kDa have been identified. Little information is so far available on the presence and nature of the chromophores on these proteins. The electron donor and acceptor specificities of the purified enzymes seem to depend to some extent on the purification procedures used. Two interesting remarks became apparent when evaluating the literature available on this subject. First, although some plasma membrane NAD(P)H-oxidoreductase activity is transmembrane, none of the purified enzymes was reported to depend on the presence of polar lipids to reach full activity. Second, considerable amounts of enzyme activity were found in the non-solubilised membrane material and apparently resisted the solubilisation procedures. The nature of these activities has not yet been clarified. Clearly the amino acid sequencing and structural analysis of these proteins will reveal important new clues to the understanding of the plasma membrane electron transport in the near future.