On the rate of translocation in vitro and kinetics in vivo of the major oxysterols in human circulation: critical importance of the position of the oxygen function

Oxysterols possess powerful biological activities. Some of their effects on the regulation of key enzymes are similar to those of cholesterol, but are much more potent. One of the critical properties of oxysterols is their ability to pass lipophilic membranes at a high rate. Transfer of unesterified 25-hydroxycholesterol from red blood cells to plasma has been reported to occur more than 1,000 times faster than cholesterol. Here we have measured the relative rate of such translocation of the three major oxysterols in human circulation: 27-hydroxycholesterol, 24S-hydroxycholesterol, and 4beta-hydroxycholesterol. The distance from the 3beta-hydroxyl group to the additional hydroxyl group is the greatest possible in 27-hydroxycholesterol and the least possible in 4beta-hydroxycholesterol. The rate of exchange between erythrocytes and plasma was found to be high for 27-hydroxycholesterol and 24S-hydroxycholesterol, and hardly possible to measure for 4beta-hydroxycholesterol and cholesterol. When injected intravenously into humans, deuterium labeled 24- and 27-hydroxycholesterol caused an immediate high enrichment of the corresponding plasma sterols followed by a decay. After injection of labeled 4beta-hydroxycholesterol, the maximum deuterium enrichment occurred after 2-3 h, when secretion of the oxysterol from the liver is likely to be the limiting factor. When radiolabeled cholesterol was injected under the same conditions, maximum appearance of label occurred after about 2 days. The results illustrate the importance of the position of the additional oxygen in oxysterols and are discussed in relation to the rate of metabolism and biological effects of these oxysterols.