Glyceride biosynthesis castalysed by the mitochondrial fraction of rat liver or by the microsomal fraction of cat intestinal mucosa is greatly stimulated by the supernatant fraction (6,000,000 ×g× min). This stimulation is due to the presence of several factors and evidence is presented that one of these factor is a phosphatidate phosphohydrolase. The catalytic activities of the phosphatidate phosphohydrolases present in the mitochondrial and supernatant fractions were studied using as subxtrates either an aqueous dispersion of phosphatidate or membrane‐bound phosphatidate formed as an intermediate in the biosynthesis of glycerides.

The mitochondrial phosphatidate phosphohydrolase has a high activity with aqueous phosphatidate dispersions and a low activity with membrane‐bound phosphatidate present as internal or added substrate. In contrast, the phosphatidate phosphohydrolase of the supernatant fraction has a low activity with aqueous phosphatidate dispersions and a high activity with membrane‐bound phosphatidate.

As Mg⁺⁺ and F⁻ ions are usually included in the assay systems employed for measuring glyceride biosynthesis, the effect of these ions on the particulate and soluble phosphatidate phosphohydrolases was studied to exclude the possibility that the stimulation of glyceride biosynthesis by the phosphohydrolase of the supernatant fraction was due to an artefact. It is suggested that the phosphatidate phosphohydrolase isolated in the supernatant fraction is the major phosphohydrolase activity involved in glyceride biosynthesis.