Presence of γ-glutamyltransferase in the renal microvascular compartment

Abstract

The association between the brush border enzyme alkaline phosphatase and γ-glutamyltransferase was determined by sucrose density gradient analysis of crude kidney homogenates, isolated glomeruli, and isolated microvessels. As previously established there is an overlap of these enzyme activities in the crude homogenate corresponding to a density of 1.17 g∙cm−3. In contrast, isolated glomeruli sedimented with a peak of 1.25 g∙cm−3 and exhibited γ-glutamyltransferase activity but little alkaline phosphatase activity; homogenizing isolated glomeruli shifted the fragments to a density coincident with that observed for the crude homogenate γ-glutamyltransferase peak. A second population of capillaries, isolated microvessels, were homogenized and analyzed on the sucrose density gradient. These fragments sedimented over the same range as crude homogenate γ-glutamyltransferase peak but were devoid of alkaline phosphatase activity and yet exhibited remarkable γ-glutamyltransferase activity. The results indicate homogenization of renal cortex results in a heterogenous collection of particles from both tubular and microvascular locations exhibiting γ-glutamyltransferase activity which overlap with the brush border alkaline phosphatase containing membranes. However, isolation of microvessels and glomeruli prior to homogenization allows separation of γ-glutamyltransferase from alkaline phosphatase activity; between 10 and 20% of the total homogenate γ-glutamyltransferase activity is estimated to be associated with the microvascular compartment.