Age-related changes in hepatic triacylglycerol content and phosphatidate phosphohydrolase activities in fasting male and female rats.

Hepatic microsomal phosphatidate phosphohydrolase (PPH) showed minor fluctuations with age in male and female rats. In males, hepatic cytosolic PPH rose to a peak at day 58 and then declined. In female rats, the activities of this enzyme were much lower than those of the male to day 35, but then increased fivefold by day 50 to values approximating those of males, and thereafter gradually declined. The hepatic triacylglycerol (TG) content of male rats was fairly constant, but in females the values rose to a peak on day 50 and then declined in parallel with the cytosolic PPH activity, but remained substantially higher than values for male rats. A significant correlation between cytosolic PPH and hepatic TG was obtained. The microsomal PPH did not correlate with hepatic TG. These results suggest that in the rat cytosolic PPH may play a role in modulating age-related changes in hepatic TG metabolism, and that it is functionally more important than the microsomal enzyme in this species.