Tritium isotope effects in the measurement of the glycerol phosphate and dihydroxyacetone phosphate pathways of glycerolipid biosynthesis in rat liver

Abstract

1. Rat liver slices were employed to study the relative rates of incorporation of a mixture of [2-3H]- or [1,3-3H]-glycerol and [1-14C]glycerol into lipids. 2. With 0.1mm-glycerol approx. 82% of the newly synthesized lipid, calculated from 14C incorporation, was present as neutral lipid, 13% as phosphatidylcholine and 5% as phosphatidylethanolamine. Increasing the glycerol concentration to 40mm caused a decrease in the percentage of neutral lipid to 59% and a corresponding increase in the percentage of phosphatidylcholine to 36% of the newly synthesized lipid. 3. The (d.p.m. of 2-3H)/(d.p.m. of 1-14C) ratio in glycerolipid was considerably higher than that in precursor glycerol throughout the range of experimental conditions. In contrast the incorporation of a mixture of [1,3-3H]glycerol and [1-14C]glycerol into lipid occurred with little or no change in the 3H/14C ratio. 4. Respiring rat liver mitochondria were found to oxidize a mixture of sn-[2-3H]- and sn-[1-14C]-glycerol 3-phosphate with a resultant increase in the 3H/14C ratio of the remaining sn-glycerol 3-phosphate. This increase is due to a 3H isotope effect of the mitochondrial sn-glycerol 3-phosphate dehydrogenase (EC 1.1.99.5), which discriminates against sn-[2-3H]glycerol 3-phosphate during oxidation. 5. A method is described for the simultaneous determination of the relative contributions of the glycerol phosphate and dihydroxyacetone phosphate pathways of glycerolipid biosynthesis in rat liver slices. The method involves measurement of the (d.p.m. of 2-3H)/(d.p.m. of 1-14C) ratio in both sn-glycerol 3-phosphate and glycerolipid after incubation of rat liver slices with a mixture of [2-3H]glycerol and [1-14C]glycerol for various times. 6. By using this method it was shown that 40–50% of the glycerol incorporated into lipid by rat liver slices proceeded via the sn-glycerol 3-phosphate pathway and 50–60% was incorporated via dihydroxyacetone phosphate.