Proteoliposome interaction with human erythrocyte membranes. Functional implantation of gamma-glutamyl transpeptidase.

The transfer of detergent solubilized and purified gamma-glutamyl transpeptidase (gamma-GTase), of hog kidney cortex, from proteoliposomes into human erythrocyte ghost membranes has been studied. The transfer of gamma-glutamyl transpeptidase was observed upon incubation of gamma-GTase incorporated dipalmitoylphosphatidylcholine vesicles with erythrocyte ghost membranes at 37 degrees C for 12 h. The extent of transfer was dependent upon the fluidity of donor proteoliposomes, being more when dipalmitoylphosphatidylcholine proteoliposomes were used compared to dimyristoylphosphatidylcholine, and intermediate values were observed when binary mixtures of DMPC and DPPC were used. Moreover, the transfer of gamma-GTase was facilitated when rigid basic phospholipid proteoliposomes were used as donor. The transfer of gamma-GTase has been observed to be associated with the removal of intrinsic membrane proteins and lipids from erythrocytes, mainly acetylcholinesterase, sphingomyelin, and cholesterol. An enhancement in the fluorescence due to resonance energy transfer was observed when ghost membranes containing fluorescent donor probe were incubated with proteoliposomes containing fluorescent acceptor probe, indicating that fusion but not adsorption of vesicles occurs during the transfer process. However, the inability of entrapped [14C]-sucrose delivery from proteoliposomes into ghost membrane vesicle suggest that fusion per se is not primarily involved in the transfer process. It appears that the transfer of gamma-glutamyl transpeptidase occurs by a collisional transfer process resulting in intermembrane protein transfer. The gamma-glutamyl transpeptidase implanted ghost membranes exhibited the uptake of L-glutamate which was inhibited by serine-borate, an inhibitor of transpeptidase activity. In addition, the uptake of L-glutamate was inhibited by the dipeptide gamma-glutamyl-L-glutamate, thus supporting the proposed role of gamma-glutamyl transpeptidase in the uptake of amino acids in biological membranes.