Guinea pig liver microsomal UDP-glucuronyltransferase: compartmented or phospholipid-constrained?

1. Three mechanical methods for disrupting membranes substantially stimulated two forward reactions and a reverse reaction of UDPglucuronyltransferase (EC 2.4.1.1.7) in guinea-pig liver microsomes. Stimulation of glucuronyltransferase was highly significant and at lease as extensive as that of nucleoside diphosphatase, reportedly a marker intracisternal enzyme. 2. Stimulation of glucuronyltransferase did not appear to be caused by induction of lipid peroxidation or by lipid hydrolysis during membrane disruption. 3. Addition of phospholipid dispersions failed to significantly re-constrain glucuronyltransferase stimulated by mechanical disruption and did not markedly inhibit the enzyme in untreated control microsomes. 4. Compartmentation appears at least as feasible an explanation for the latency of glucuronyltransferase in guinea-pig liver microsomes as is its conformational constraint by membrane phospholipids. Compartmentation of glucuronyltransferase is functionally attractive since it would ensure its effective interaction with nucleoside diphosphatase on the cisternal side of the endoplasmic reticulum. This would tend to make conjugation reactions unidirectional.