Patterns of cystine reduction by fibroblasts from normal and cystinotic children

The possibility that the enzymatic reduction of cystine involves a multi-enzyme system led to re-evaluation of cystine reduction by fibroblasts from normal and cystinotic patients. Lineweaver-Burk plots of data with extracts of a normal cell line, representative of seven normal cell lines, under conditions of increasing cystine with variable levels of reduced glutathione (GSH) resulted in a two-limbed curve above 100 micronM cystine. Two cell lines from children with nephropathic cystinosis containing 1-6 micronmol half-cystine/g protein gave a family of curves similar to those of the normal. With six fibroblast lines containing more than 6 micronmol half-cystine/g protein, increasing cystine resulted in a family of lines without two-limbed curves. Plots of the data as activity against increasing cystine concentrations at ratios of cystine to GSH of 1:2 and 1:1 showed that two of the three lines from cystinotic subjects reduced cystine at a faster rate than the normal line. The third line from a cystinotic patient reduced cystine at a slightly slower rate when the substrate concentration in the assay was less than 80 micronM cystine. When the cystine to GSH ratio was maintained at 2:1, normal cells showed a linear increase in the rate of cystine reduction up to 100 micronM cystine, no increase in the rate between 100 micronM and 200 micronM cystine, and an increase again when the concentration of cystine was raised above 200 micronM. Such a stepwise phenomenon was absent with six cell lines containing more than 6 micronmol half-cystine/g cellular protein. A possible mechanism of control of cystine reduction is discussed.