Cystathionine occurrence

Our findings in this study are in harmony with the concept that L-cystathionine is an intermediate in the formation of cystine from methionine in man. Direct evidence for the existence of cystathionine in man was provided by the demonstration by Tallan, Moore, and Stein (13) of the occurrence of L-cystathionine in extracts of human brain. Moreover, cases of human cystathioninuria have been reported by Harris, Penrose, and Thomas (9) and by Frimpter, Haymovitz, and Horwith (8). The latter authors have also stated that an increased renal clearance of cystathionine is not observed in cystinuria. It is of considerable interest, however, that the mixed disulfide of L-cysteine and L-homo-... 

The evidence given above for the key role of cystathionine in the transsulfuration reaction to form cystine from methionine would be measurably strengthened by proof of the occurrence of this compound as a product of the metabolism of methionine. In Neurospora cystathionine has been isolated from the culture medium of certain methionineless mutants and shown to be a step in the conversion of cysteine to methio-... 

Greatly increased lactic aciduria is generally associated with an increased excretion of 2-hydroxy-n-butyric acid (Pettersen et al., 1973 Landaas and Pettersen, 1975) irrespective of the underlying cause of the increased lactate excretion. This acid is thought to arise from 2-oxobutyric acid, a metabolite of methionine, homocystine, cystathionine, serine and threonine, owing to the effect of the changes in NADH/NAD+ ratio in lactic acidosis (Pettersen etaL, 1973). The occurrence of 2-hydroxybutyrate in urine would thus imply an imbalance in cytosolic NADH/NAD and this metabolite may be of value in the further assessment of patients with lactic acidosis. 

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