Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Metabolism of Simple Proteins
Absorption of Protein Breakdown Products
Transport of Amino Acids Across Cell Membranes

The varying rates of amino acid penetration through Cell membranes, established using the radioactive tracer method, indicate the existence of an Active Transport system in the Organism that ensures The transfer of Amino Acids both across the outer Plasma Membrane and through the intracellular membrane system. Despite thorough research conducted in various laboratories, the subtle mechanisms governing the active Amino Acid Transport system remain undecoded. Evidently, several such systems exist. In particular, A. Meister proposed an original scheme for The transport of neutral amino acids across The Plasma Membrane, which appears to be active in the renal tubules, the intestinal mucosa, and A number of other Tissues. The Essence of this hypothesis can be represented in the following diagram:

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It is hypothesized that the primary role in this process is played by a membrane-bound glycoprotein—the enzyme Y-glutamyltransferase, which catalyzes the transfer of the Y-glutamyl group from Glutathione or another Y-glutamyl peptide to The amino acid being transported. After crossing the biomembrane, the Y-glutamyl-amino acid complex dissociates inside The Cell (or within a subcellular Structure) under the action of Y-glutamylcyclotransferase into a free Amino Acid and 5-oxoproline (pyroglutamic acid), The formation of which almost entirely shifts the Cleavage reaction of the complex to the right. Due to the possibility of glutathione resynthesis, which requires the expenditure of ATP energy, the cycle can repeat multiple times, transporting significant quantities of amino acids.