[Does a modified gamma-glutamyl cycle exist in human erythrocytes (author's transl)]

Area of Science:

• Biochemistry
• Cellular Metabolism

Background:

• Glutathione biosynthesis begins with gamma-glutamyl-cysteine formation by glutamyl-cysteine synthetase.
• Non-specificity of glutamyl-cysteine synthetase leads to various gamma-glutamylamino acids, potential substrates for gamma-glutamylcyclotransferase.
• Gamma-glutamylcyclotransferase activity produces 5-oxo-L-proline and free amino acids.

Purpose of the Study:

• Investigate the competition between glutathione/ophthalmic acid biosynthesis and gamma-glutamyl peptide degradation.
• Quantify 5-oxo-L-proline formation as an indicator of this metabolic competition.
• Elucidate the role of different amino acids in these pathways.

Main Methods:

• Utilized membrane-free hemolysate to study metabolic pathways.
• Measured the formation rate of 5-oxo-L-proline under various conditions.
• Assessed the impact of adding amino acids like 2-aminobutyrate, glutamate, and cysteine on 5-oxo-L-proline production.

Main Results:

• Endogenous 5-oxo-L-proline production rate was 0.13 muM/min.
• Addition of 2-aminobutyrate increased production to 2 muM/min; glutamate and 2-aminobutyrate increased it to 10 muM/min.
• Degradation of gamma-glutamyl-2-aminobutyrate to 5-oxo-L-proline was faster than its use in tripeptide synthesis, unlike gamma-glutamyl-cysteine.

Conclusions:

• 5-oxo-L-proline formation in membrane-free hemolysate suggests an internal pathway independent of gamma-glutamyltransferase.
• The results indicate a competition between peptide synthesis and degradation pathways.
• A modified gamma-glutamyl cycle may be involved in amino acid transport.