Cytoplasmic gamma-glutamyltransferase: isolation, product formation and physiological role.

Abstract

These results establish the existence of a cytoplasmic glutaminase-gamma-glutamyltransferase enzyme as a distinct entity. Its products are ammonia and activated glutamate. Ammonia liberation is obligatory to the utilization of the amide bond energy in forming gamma-glutamyl-PO4. This activated glutamate can then be utilized in the gamma-glutamyl cycle for the synthesis of gamma-glutamylcysteine. The cytoplasmic glutamine utilizing pathway is closely coupled to gamma-glutamyl cycl activity: loading the cycle stimulates increased renal glutamine uptake into this pathway. Consequently, the pathway, stimulated by elevated ADP levels, appears to function as an auxilary source of gamma-glutamyl moieties for the gamma-glutamyl cycle. Although insignificant compared to the mitochondrial pathway's contribution to ammonia production in metabolic acidosis, it is highly significant from the perspective of the Unitary Hypothesis. The existence of this dual system allows the demonstration of a shift in glutamine utilization from predominant cytoplasmic to overwhelming mitochondrial glutamine utilization in metabolic acidosis corresponding to a rise in the NH3/gln ratio from 1.2 to 1.9 and a quantitative recovery of gln carbon as CO2 and glucose. The fact that this shift in pathways is induced by acidosis (through adrenosteroids) and that it represents a 10 to 20 fold activation of the mitochondrial pathway is completely consistent with a glucocorticoid mediated glutamine permeability increase of the inner mitochondrial membrane.