Disturbances of cystine metabolism in liver disease.

The urinary excretion of cystine, as determined on the polarograph, varies, in normal adults, between 40 and 80 mg. daily (1). It may be increased either as a result of a low renal threshold for cystine or a raised plasma cystine concentration. Cystinuria due to a low renal threshold occurs in 'classical cystinuria,' the Fanconi syndrome and hepatolenticular degeneration (2). In such conditions the excretion of cystine is usually large, 0.5 to 1.0 g. daily, and is associated with other characteristic changes in the pattern of amino acids excreted. A moderately increased excretion of cystine has been described in patients with liver disease (3-5) and this has been shown, by paper chromatography, to result from a raised plasma cystine concentration. It has been suggested that the increased urinary cystine in these patients might form a sensitive index of impaired amino acid metabolism (6). Using the quantitatively more accurate method of polarography for cystine assay we have reinvestigated the plasma level and urinary output of cystine in cases of liver disease. In addition we have carried out cystine and cysteine tolerance tests comparing the results found in patients with chronic liver injury with those found in normal subjects.