Triacylglycerol synthesis in developing seeds of groundnut (Arachis hypogaea): pathway and properties of enzymes of sn-glycerol 3-phosphate formation

The enzymatic pathway for the synthesis of sn-glycerol 3-phosphate was investigated in developing groundnut seeds (Arachis hypogaea). Glycerol-3-phosphate dehydrogenase was not detected in this tissue but an active glycerokinase was demonstrated in the cytosolic fraction. It showed an optimum pH at 8.6 and positive cooperative interactions with both glycerol and ATP. Triosephosphate isomerase and glyceraldehyde-3-phosphate phosphatase were observed mainly in the cytosolic fraction while an active glyceraldehyde reductase was found mainly in the mitochondrial and microsomal fractions. The glyceraldehyde 3-phosphate phosphatase showed specificity and positive cooperativity with respect to glyceraldehyde 3-phosphate. The glyceraldehyde reductase was active toward glucose and fructose but not toward formaldehyde and showed absolute specificity toward NADPH. It is concluded that in the developing groundnut seed, sn-glycerol 3-phosphate is synthesized essentially by the pathway dihydroxyacetone phosphate----glyceraldehyde 3-phosphate Pi----glyceraldehyde NADPH----glycerol ATP----glycerol 3-phosphate. All the enzymes of this pathway showed activity profiles commensurate with their participation in triacylglycerol synthesis which is maximal during the period 15-35 days after fertilization. Glycerokinase appears to be the rate-limiting enzyme in this pathway.