Comparative studies of the responses of rat liver microsomal glucose-6-phosphatase and inorganic pyrophosphate-glucose phosphotransferase to phospholipase C treatment and phospholipid supplementation

Abstract

1. 1.|Comparative studies of the responses of Glc-6- P phosphohydrolase and PP i-glucose phosphotransferase activities of rat liver microsomal glucose-6-phosphatase (EC 3.1.3.9) to in vitro phospholipid modifications have been carried out. Various conditions for assay enzymic activities were employed, and results compared. 2. 2.|Based on activity assays performed at pH 6 in the absence of added detergent, both activities decreased progressively and in parallel fashion with increasing length of exposure of microsomes to the action of phospholipase C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3). These activities of such treated preparation were increased by the addition of micellular dispersions of supplemental phospholipid (rat liver microsomal phospholipid preparations or, in one study, various classes of purified phospholipids). These observations provide yet another piece of evidence in support of catalysis of these two enzymic activities by a single microsomal enzyme. 3. 3.|The degree of restoration of activities towards original control levels through phospholipid supplementation following phospholipase C exposure of microsomes was much less when based on activity values obtained in the presence of the detergent deoxycholate than in its absence. 4. 4.|Enzymic activities of phospholipid-supplemented, phospholipase C-treated preparations were quite insensitive to stimulation by deoxycholate, while those of unexposed preparations were highly stimulated by this same detergent. 5. 5.|The pH-activity profile for PP i-glucose phosphotransferase activity obtained after phospholipase C exposure and phospholipid supplementation resembled strongly that noted with detergent-activated, otherwise untreated microsomal preparations, displaying a maximum at pH 5.6 in comparison with that at pH 4.5 exhibited by untreated microsomal preparations. 6. 6.|These observations are interpreted to indicate that phospholipid supplementation following phospholipase C exposure of rat liver microsomes does not lead to the restoration of microsomal glucose-6-phosphatase in its original form. Further, it is concluded that, in view of these findings and other observations in the literature, a reconsideration of the whole subject of the involvement of phospholipids in glucose-6-phosphatase action appears to be in order.