SAPOSIN (SAP) A AND SAPOSIN-C ACTIVATE THE DEGRADATION OF GALACTOSYLCERAMIDE IN LIVING CELLS

In loading tests using galactosylceramide which had been labelled with tritium in the ceramide moiety, living skin fibroblast lines derived from the original prosaposin-deficient patients had a markedly reduced capacity to degrade galactosylceramide. The hydrolysis of galactosylc eramide could be partially restored in these cells, up to about half t he normal rate, bg adding pure saposin A, pure saposin C, or a mixture of these saposins to the culture medium, By contrast, saposins B and D had little effect on galactosylceramide hydrolysis in the prosaposin -deficient cells, Cells from beta-galactocerebrosidase-deficient (Krab be) patients had a relatively high residual galactosylceramide degrada tion, which was similar to the rate observed for prosaposin-deficient cells in the presence of saposin A or C. An SV40-transformed fibroblas t line from the original saposin C-deficient patient, where saposin A is not affected, showed normal degradation of galactosylceramide. The findings support the hypothesis, which was deduced originally from in vitro experiments, that saposins A and C are the in five activators of galactosylceramide degradation. Although the results with saposin C-d eficient fibroblasts suggest that the presence of only saposin A allow s galactosylcesamide breakdown to proceed at a normal rate in fibrobla sts, it remains to be determined whether saposins A and C can substitu te for each other with respect to their effects on galactosylceramide metabolism in the whole organism. (C) 1997 Federation of European Bioc hemical Societies.