James L. Gaylor

Microsomal enzymes of sterol biosynthesis.

Publisher Summary Microsomes are prepared by differential centrifugation of broken-cell preparations. The microsomes result from cellular endoplasmic reticulum that has been shown to be the site of cholesterol biosynthesis in mammalian liver cells. The reticulum develops at early stages of fetal life but not all enzymatic, electron carrier, and lipid components are present in microsomes isolated from neonatal livers. Evidence shows that at the time of birth, 80% to 85% of the cholesterol is of fetal and not maternal origin. Thus, the full complement of microsomal enzymes of sterol biosynthesis is present in neonatal liver. Various marker enzymes and pigments have been used to distinguish microsomes from preparations of other particles; similar investigations have shown that microsomal membranes might be distinguished from membranes of other cellular organelles. This chapter focuses on the enzymes of sterol biosynthesis and closely related enzymatic processes.

Investigation of the multienzymic system of microsomal cholesterol biosynthesis.

We are studying the multienzymic process of the biosynthesis of cholesterol from lanosterol. Chesterton’ showed that these reactions are catalyzed by enzymes that are part of the endoplasmic reticulum of the liver cell, and that the multienzymic system is bound to isolated microsomal membranes in cell-free preparations. All of the enzymes needed for the conversion of lanosterol to cholesterol are present in the microsomal fraction obtained from rat liver and other tissues. The reactions that occur in the conversion of lanosterol to cholesterol are shown diagrammatically in FIGURE 1. As summarized by Bloch,’ the A”-double bond of lanosterol is reduced by an NADPH-dependent enzyme, the A“’”’-double bond of lanosterol is removed, and a A5-double bond is introduced in the forrnation of cholesterol. Bloch and Colleagues“ first showed that each of the three “extra” methyl groups of lanosterol, 30-C, 31-C, and 32-C, is converted to carbon dioxide by aerobic enzymes that require both reduced and oxidized pyridine n ucieotides.

Further characterization of the 5α-hydroxysterol dehydrase of yeast

Under conditions now established for measurement of the rate of dehydration, ergosterol has been shown to be formed at equal rates from 5α,8α-epidioxyergosta-6,22-dien-3β-o1 and ergosta-7,22-diene-3β,5α-diol substrates. Furthermore, 3α-3HH is lost during formation of ergosterol from the 5α-hydroxysterol substrate. Thus, stepwise loss of the leaving group from the 5α position, followed by stabilization of an intermediate by loss of the α-proton from 3-C, is proposed.