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Metabolism of neutral glycosphingolipids in plasma of a normal human and a patient with Fabry's disease.
[6-2H2]Glucose was used as a tracer for a comparative study on the metabolism of the neutral glycosylceramides in plasma of a control subject and a patient with Fabry's disease. The incorporation of the tracer into the glucosyl and galactosyl moieties of the glycosphingolipids was measured by gas chromatography-mass spectrometry of the tetra-0-acetyl methyl glycoside derivatives. Experiments on the precision and accuracy for measurements of [6,6-2H2]hexose in a sample demonstrated that incorporation of 0.2% or more of [6,6-2H2]glucose could be detected with a 95% confidence limit of +/-0.16%. The labeled substrate (35g) was infused into each subject with a 5-g priming dose and the remainder administered at a constant rate of 3 g/hour over a 10-hour period. During the infusion, the plasma glucose of each subject attained a concentration of about 30% [6,6-2H2]glucose which diminished rapidly after the administration of substrate was complete. A concentration of 0.8% [6,6-2H2]glucose was observed in glucosylceramide (GL-la) from plasma of both subjects between 48 and 72 hours after the infusion began. The label disappeared from this lipid at a logarithmic rate and 0.2% or less of the molecules were labeled 9 days after the experiment began. In contrast to the results with GL-la, the maximum incorporation of [6,6-2H2]hexose into lactosylceramide (galactosyl-(beta1 leads to 4)-glucosylceramide) was 2-fold higher in the Fabry patient (1.6%) than in the control (0.8%). The trihexosylceramide (galactosyl-(alpha1 leads to 4)-galactosyl-(beta1 leads to 4)-glucosylceramide, GL-3a) from plasma of the control reached a maximum of 0.4% [6,6-2H2]hexose in both the glucosyl and galactosyl moieties whereas the GL-3a from the Fabry patient was not significantly labeled. The maximal labeling of the GL-4a fraction (N-acetyl-galactosaminyl-galactosyl-galactosyl-glucosylceramide) was slightly depressed in the Fabry patient (0.4%) as compared to the control (0.7%). Turnover times for the glycosphingolipids of plasma were calculated to be from 4 to 8 days and the turnover rates were from 1 to 6 mumol/day.
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