The metabolism of
galactosylceramide and
lactosylceramide in cultured fibroblasts was studied using the
lipid-loading test. These compounds were incorporated into the fibroblasts yet only small amounts of the incorporated
lipids were hydrolyzed unless additional
phospholipid was mixed with the
glycolipid before loading. Among
phospholipids,
phosphatidylserine was the most effective for incorporation and hydrolysis of the
glycolipids, while
phosphatidylcholine inhibited the incorporation of the
glycolipids. Using filtration techniques, light scattering analyses and subcellular fractionation, the particle size of
glycolipid in the culture medium was found to be critically important for the incorporation of the
lipids into the cells and their transportation to the lysosomes. The particle sizes of the
glycolipids were decreased by mixing with
phosphatidylserine. Furthermore, the negative charge in
phosphatidylserine may be necessary for the
glycolipid transportation into the lysosomes. In fibroblasts from patients with
globoid cell leukodystrophy, 40-50% of
galactosylceramide was hydrolyzed on the 4th day of culture, a time when the control fibroblasts had hydrolyzed it about 80%. This finding is in contrast with observations made on fibroblasts with other
sphingolipidoses which showed near-zero degradation in corresponding substrate-loading tests. In fibroblasts from patients with either
globoid cell leukodystrophy of GM1-gangliosidosis, hydrolysis of
lactosylceramide was fairly normal yet somewhat lower than control values on any day of culture, thereby indicating that, in the loading tests,
lactosylceramide seems to be hydrolyzed with similar levels of
enzyme activities by two distinct
beta-galactosidases,
galactosylceramidase and GM1-ganglioside
beta-galactosidase.