Properties of the enzymes catalyzing the biosynthesis of lysophosphatidate and its ether analog in cultured fibroblasts from Zellweger syndrome patients and normal controls

Abstract

The activities, properties, and steady-state kinetics of the five enzymes catalyzing the synthesis of 1-acyl- and 1-alkyl- sn-glycerol 3-phosphate in the cultured skin fibroblasts from Zellweger syndrome patients and normal controls were studied in detail. Judging from their K m and V max values, glycerol phosphate acyltransferase (EC 2.3.1.15), acyl/ alkyl dihydroxyacetone phosphate reductase (EC 1.1.1.101), and acyl coenzyme A reductase (long-chain alcohol forming), appear to be affected only slightly by the absence of peroxisomes characteristic of the Zellweger syndrome. Glycerophosphate acyltransferase also showed no differences in N-ethylmaleimide sensitivity nor in inhibition by dihydroxyacetone phosphate between these cell types. Dihydroxyacetone phosphate acyltransferase (EC 2.3.1.42) and alkyl dihydroxyacetone phosphate synthase (EC 2.5.1.26) have altered activity and kinetic constants in homogenates from Zellweger syndrome fibroblasts. Dihydroxyacetone phosphate acyltransferase has similar K m (DHAP) values in both control and Zellweger syndrome cells; however, the value for the V max in Zellweger syndrome cells is only 6% of that found in the controls. This is interpreted as indicating that this enzyme is not defective in this disease but is simply present at a depressed level. Also, this enzyme activity has a maximum rate at pH 7.0–7.5 in the mutant cells as opposed to pH 5.4 in the controls. Acylation of dihydroxyacetone phosphate by control cell homogenate was stimulated by N-ethylmaleimide at both pH 5.7 and 7.5 whereas this activity from Zellweger syndrome cells was slightly inhibited at pH 5.7 and strongly inhibited at pH 7.5. In the absence of detergent, dihydroxyacetone phosphate acyltransferase in the Zellweger syndrome cells was much more labile to trypsin than in the control cells. Alkyl dihydroxyacetone phosphate synthase had a slightly higher K m (33 vs 17 μ m) for palmitoyl dihydroxyacetone phosphate and a lower V max (0.07 vs 0.24 mU/mg protein) in the Zellweger syndrome cells as compared to controls. Although this is a substantial decrease in activity, it probably contributes little to the decreased rate of ether lipid synthesis in these cells. The major problem in this respect is apparently the loss of dihydroxyacetone phosphate acyltransferase activity. All of these enzymes, in both control and Zellweger syndrome cell homogenates, are sedimentable by centrifugation at 100,000 g. Also, with the exception of dihydroxyacetone phosphate acyltransferase they had similar patterns of inactivation by heat in both cell types.