The localization of gamma-glutamyl transpeptidase (γ-GTP) in the rat kidney was examined by immunoelectron microscopy using monoclonal antibody to rat γ- GTP. The enzyme was distributed along the brush border and in the apical canaliculi and vacuoles of the proximal tubules. The immunoreaction of γ-GTP in the brush borders increased from the S1 to S3 segments in the proximal tubules. Conspicuousγ;-GTP immunostaining was observed along the basolateral membranes of the S2 and S3 segments of the proximal tubules. In addition, the apical cell membrane of the upper part of descending thin limbs of Henle's loops exhibited positive γ-GTP immunostaining. A faint reaction was occasionally recognized in the apical cell membranes of the distal convoluted cells, including the macula densa.
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