Expression and activity of gamma‐glutamyl transpeptidase in the rat epididymis

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Abstract
Following Northern analysis, GGT mRNA was found predominantly within the caput epididymides and kidney. The size of mRNAs for kidney, caput, corpus, and ducts deferens were 2.2, 2.3, 2.2., and 2.3 kb, respectively, whereas cauda showed a doublet of 2.2 and 2.3 kb. GGT transpeptidation and hydrolytic activity within epididymal luminal fluids collected by micropuncture showed caput=corpus>cauda and corpus>caput>cauda, respectively. Caput luminal GGT transpeptidation activity was significantly inhibited by serine-borate and was optimal at pH 8.0. The calculated Km and Vmax values for hydrolysis of GSH by caput luminal GGT were 0.06 μM and 2.19 nmoles/min/μl luminal fluid at pH 8.5 compared to 0.49 μM and 0.49 nmoles/min/μl luminal fluid, respectively, at the physiological pH 6.5 of caput fluid. These studies would suggest that the epididymis can control the activity of luminal GGT by pH. Lower Km (0.12 μM) and higher Vmax (1.13 nmoles/min/μl luminal fluid) values were also calculated when GSSG was used compared to GSH. Results from Triton X-114 partitioning experiments suggest that luminal GGT probably exists in both membrane bound and nonmembrane bound forms. Western blot analysis of proteins within epididymal luminal fluids revealed both subunits of GGT in all epididymal regions studied. However, two lower molecular bands, approximately 22 kDa and 21 kDa, were also observed in cauda fluid. It is suggested that as GGT is transported along the epididymal duct it undergoes degradation, which accounts for its loss of activity in the distal epididymal regions. Epididymal GGT may not be involved in the transport of L-glutamate since transport was not related to the degree of GGT mRNA expression along the epididymal duct.