Trypsin‐susceptibility of UDP‐glucuronyltransferase

Abstract

The location in liver microsomes of the membranebound enzyme system, UDP-glucuronyltransferase (EC 2.4.1.17) is controversial (e.g., [l-5]). Findings of Finnish workers [ 1,2,6-81 suggested that the transferase was not exposed on the cytoplasmic face of rat liver microsomal membranes, since tryptic digestion of ‘intact’ microsomes stimulated it, while trypsin strongly inhibited in disrupted microsomes. However others have reported that trypsin had little or no effect upon the transferase in rat [9111 or guinea pig liver microsomes [5] , or that it inhibited the transferase in ‘intact’ guinea pig liver microsomes [3,121 We have argued on different grounds that glucuronyl-transferase is probably structurally latent [4,13,18] which is also the view of others (e.g., [ 1,2]). However, since some trypsin data do not fully support latency [ 3,5,9121 and have been cited as evidence against it [5], the effects of trypsin upon the transferase were further studied in ‘intact’ and disrupted microsomes from both rat and guinea pig liver. Trypsin moderately stimulated the transferase in ‘intact’ microsomes from both species, but it greatly inhibited in disrupted microsomes.