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Abstract:
Pinocytic vesicles, brush border microvilli, lysosomes and basolateral plasma membranes were isolated from rat kidney cortex and their biochemical composition and membrane turnover compared. Pinocytic vesicles are devoid of marker enzymes of brush border microvilli, such as alkaline phosphatase and 5′-nucleotidase, and of lysosomes, such as acid phosphatase and β-glucuronidase. The protein pattern as revealed by polyacrylamide gel electrophoresis differs for all four membranes. Analysis of the phospholipid composition shows that pinocytic vesicles are rich in the negatively charged phospholipid phosphatidylserine and have a low content of sphingomyelin and phosphatidylethanolamine.
[14C]guanido-arginine, [3H]fucose and myo-[3H]inositol were preferentially incorporated into the pinocytic vesicles. Using a double label technique with leucine also, evidence of a more rapid turnover of the pinocytic vesicle membrane proteins was obtained.
The results suggest that pinocytic vesicles are not derived from the brush border microvillous membrane but are independent entities that are newly synthesized during the pinocytic process.