Methionine, cystathionine and cystine increased urinary losses during total parenteral-nutrition of adult patients

Amino-acids involved in transsulphuration pathway were studied in the plasma and urine of 49 depleted surgical patients and 36 critically ill, injured patients who received totalparenteralnutrition. A minimum B6 intake of 5 mg day−1 was provided to all patients; nitrogen intake was supplied with different amino-acid solutions. A daily methionine load higher than 15–24 mg kg−1, resulted in increasedmethionine plasma levels and increasedurinaryloss, both linearly related to methionine intake and notwithstanding the metabolic status of the patients. The increase of methionine load caused the appearance of traces of cystathionine in plasma and the increase of cystathionine urinaryloss that were always higher than reference. This increase was linearly related to methionine intake in both patient groups, the slope being significantly higher in the depleted group. It suggests a limiting role of cystathioninase in the transsulphuration pathway. Its activity seems lower in depleted than in catabolic patients. During totalparenteralnutrition, in spite of lower than normal cystine plasma values, cystineurinary output was always 2–10 times higher than reference in both patient groups, although cystineurinaryloss was linearly related to methionine intake. Taurine and amino-butyrate were not significantly affected by totalparenteralnutrition