THE BIOSYNTHESIS OF CHOLESTEROL AND OTHER STEROLS BY BRAIN TISSUE: DISTRIBUTION IN SUBCELLULAR FRACTIONS AS A FUNCTION OF TIME AFTER INJECTION OF [2-14C]MEVALONIC ACID, SODIUM [2-14C]ACETATE AND [U-14C]GLUCOSE INTO 15-DAY-OLD RATS

The distribution of ¹⁴C]-labelled material into subcellular fractions of 15-day-old rat brain was studied at 2 and 24 h following intraperitoneal and intracerebral injection of 2-¹⁴C]sodium acetate, U-¹⁴C]glucose and 2-¹⁴C]mevalonic acid respectively. The total quantity of labelled isoprenoids in the brain was, except for glucose, greater when the precursor was administered intracerebrally. The intraperitoneal route was more advantageous in the case of U-¹⁴C]glucose. The subcellular distribution of both labelled total isoprenoid material and sterol was distinct for each labelled precursor. Intracerebrally injected U-¹⁴C]glucose at both time periods studied suggested no dominance of labelling in any fraction. After intraperitoneal injection of U-¹⁴C]glucose the microsomes were more prominently labelled. Both methods of administration of sodium 2-¹⁴C]acetate resulted in heavy labelling of the myelin fraction after 24 h. The total labelled isoprenoids resided mainly in the microsomes 24 h after injection of 2-¹⁴C]mevalonic acid. Labelled sterol was found to be localized more in the myelin and microsomal fractions for all three precursors than was the labelled total isoprenoids. Depending on the type of experiment to be conducted, each of these precursors can give different results, which must be interpreted accordingly.