The role of a 5alpha-hydroxylated intermediate in the formation of the 5, 6-double bond in cholesterol biosynthesis

If the biological conversion of cholest-7-en-3beta-ol (I) into cholesterol (IV) occurred thorugh the intermediacy of cholest-7-ene-3beta,5alpha-diol (II) then the factor(s) adversely affecting the convwesion of the 5alpha-hydroxy sterol (II) into cholesterol must at least equally adversely affect the formation of cholesterol from cholest-7-en-3beta-ol. By using partial denaturation techniquws and dual-labelled precursors it was shown that the enzyme system responsible for the conversion of the 5alpha-hydroxy sterol (II) into cholesterol denatured faster than that for the corresponding conversion from cholest-7-en-3beta-ol (I).

The sterols of the echinoderm Asterias rubens

1. Twenty-two sterols were identified in the starfish Asterias rubens (Phylum, Echinodermata; Class, Asteroidea). 2. The major 4-demethyl sterols had a Delta(7) bond and the C(27) compound 5alpha-cholest-7-en-3beta-ol predominated over other mono- and di-unsaturated sterols belonging to the C(26), C(27), C(28) and C(29) series. 3. Small amounts of cholest-5-en-3beta-ol and 5alpha-cholestan-3beta-ol were also present. 4. The minor sterols identified all contained either one or two methyl groups at C-4 and are considered to be potential biosynthetic precursors of 5alpha-cholest-7-en-3beta-ol. 5. Three sterols possessing a 9beta,19-cyclopropane ring were also isolated and were probably derived by the starfish from a dietary source.

The formation of cholest-5-ene-3 ,26-diol as an intermediate in the conversion of cholesterol into bile acids by liver mitochondria

1. When [(14)C]cholesterol was incubated with rat liver mitochondria, radioactive 26-hydroxycholesterol, 3beta-hydroxychol-5-enoic acid and other bile acids were isolated from the incubation mixture. 2. In the absence of added 26-hydroxycholesterol, the specific radioactivity of the 26-hydroxycholesterol formed from [(14)C]cholesterol during the incubation was higher than that of the 3beta-hydroxychol-5-enoic acid. Addition of increasing amounts of 26-hydroxycholesterol led to a progressive fall in the specific radioactivity, and to a progressive increase in the mass, of the 3beta-hydroxychol-5-enoic acid recovered at the end of the incubation. 3. It is concluded that 26-hydroxycholesterol is an intermediate in the formation of 3beta-hydroxychol-5-enoic acid from cholesterol. 4. Comparison of the specific radioactivity of the 26-hydroxycholesterol formed in the incubation mixture with that of the added [(14)C]cholesterol indicates that endogenous cholesterol in mitochondria is accessible to cholesterol 26-hydroxylase.

Hydrogen exchange and double bond formation in cholesterol biosynthesis

The conversion of lanosterol║to cholesterol requires a considerable number of intermediary steps involving loss or uptake of hydrogen atoms and formation and migration of nuclear double bonds. Detailed discussions on the intermediary steps in cholesterol biosynthesis are reported in several reviews (Olson 1965; Frantz & Schroepfer 1967; Goad 1970). In the present report some mechanisms in the formation of cholesterol and its sterol precursors from lanosterol are discussed. The relation between in vitro and in vivo pathways of cholesterol biosynthesis and the composition and metabolism of sterols in biological tissues is underlined.

Substrate activation in pyridine nucleotide-linked reactions: illustrations from the steroid field

The work on the mechanism of reduction of steroid 7(8) 14(15) and 24(25) double bonds is summarized. On the basis of the results obtained it is suggested that the first crucial event in the pyridine nucleotide-linked reduction is the activation of the substrate through protonation to give an electron deficient species which in the next step is neutralized by the addition of a hydride from NAD(P)H to furnish the product.

The sterols of echinoderms

The pioneering work of Bergmann and his colleagues demonstrated that marine invertebrates in many cases contain complex sterol mixtures consisting of C 27 , C 28 and C 29 sterols of varying degrees of unsaturation (Bergmann 1949, 1962). The sterols found in the phylum Echinodermata have proved of particular interest not only from the point of view of their composition and biological origin but also from phylogenetic considerations. Dorée (1909) first recognized that the sterol of a starfish differed from cholesterol, the typical sterol of higher animals. Later Kossel & Edlbacher (1915) obtained the sterol from a starfish, Asteropecten aurantiacus , and named it stellasterol. Subsequently Bergmann & Stansbury (1944) concluded that starfish sterols are mixtures which include stellastenol (1) and stellasterol, the 22, 23-dihydro derative of stellastenol (Shoppee 1964) Extending their studies Bergmann and co-workers demonstrated that the echinoderms can be divided into two groups on the basis of the types of sterol which they contain. Crinoidea (sea lilies), Ophiuroidea (brittle stars) and Echinoidea (sea urchins) contain sterols with a ∆ 5 bond while the Asteroidea (starfish) and Holothuroidea (sea cucumbers) contain mixtures in which sterols with a ∆ 7 bond predominate (Bergmann 1962). This division of echinoderms according to sterol type has recently been confirmed by Gupta & Scheuer (1968) using more modern methods of sterol analysis. Bergmann (1962) concluded that the distribution of ∆ 5 and ∆ 7 sterols in the different classes of echinoderms is a reflexion of the phylogenetic relationships which exist in the phylum. From embryological evidence there is a close relationship between the asteroids and holothuroids on the one hand and the ophiuroids and echinoids on the other (Bergmann 1962; Hyman 1955). In addition to the data on sterols other comparative biochemical evidence on the occurrence and distribution of batyl alcohol, spinochromes and phosphagens in echinoderms is in accord with these phylogenetic relationships (Singh, Moore & Scheuer 1967; Bolker 1967 α ). The validity of the embryological and biochemical evidence, however, has been contested by Fell who regards the asteroids to be more closely related to the ophiuroids while the holothuroids have some affinities with the echinoids (Fell 1948, 1963, 1965; Fell & Pawson 1966; Bolker 1967 α ).

Biosynthesis of 5 -cholestan-3 -ol in cerebrotendinous xanthomatosis

Cerebrotendinous Xanthomatosis is a rare, inherited disease characterized by an extraordinary accumulation of cholestanol in all tissues, xanthomatous deposits in the brain, lungs, and Achilles tendons, premature atherosclerosis, and low plasma cholesterol concentrations. In two patients with the disease, the biosynthesis of cholestanol was examined by different techniques. After cholesterol-4-(14)C was injected intravenously into one patient, cholestanol and cholesterol isolated from the bile on 3 different days over the ensuing week contained significant radioactivity. The specific radioactivity-time curves for cholesterol-(14)C and cholestanol-(14)C suggested a precursor product relationship and provided additional evidence for the transformation of cholesterol into cholestanol. The second patient received intravenously a mixture of mevalonate-2-(14)C and stereospecifically labeled mevalonate-3R,4R-(3)H. Again cholesterol and cholestanol were isolated from the bile, and the (3)H/(14)C ratio in both sterols was almost the same. This experiment again demonstrated that the biosynthetic path of cholestanol proceeded through cholesterol and not directly from earlier 5alpha-H-saturated precursors. These two independent lines of evidence indicate that the extraordinary deposition of cholestanol in Cerebrotendinous Xanthomatosis arises from cholesterol presumably through the accentuation of the normal biosynthetic pathway.

Enzymic formation of esters of methyl sterol precursors of cholesterol

For investigation of the reactions of cholesterol biosynthesis, a number of workers use the 10,000 g supernatant fraction (or similar preparations) obtained from cell-free homogenates of rat liver. We have found that esters of methyl sterol biosynthetic intermediates are formed by this crude source of enzymes. Esters of C(30)-, C(29)-, C(28)-, and C(27)-sterol intermediates have been isolated by silicic acid chromatography of an acetone extract of incubation mixtures. Competition between ester formation and demethylation of the C(28)-sterol intermediate has been demonstrated. With 4alpha-methyl-5alpha-cholest-7-en-3beta-ol as substrate, maximal velocities of ester formation (0.36 nmole/30 min per mg of protein) were almost equivalent to maximal velocities of demethylation (0.45 nmole/30 min per mg of protein). Ester formation may be eliminated by carrying out incubations with microsomal preparations; ester formation may be restored completely upon addition (to the microsomes) of either coenzyme A and ATP or the supernatant fraction resulting from centrifugation at 105,000 g. Ester formation has been examined similarly with broken-cell preparations of rat skin. With $$Word$$ as substrate, the rate of ester formation was more than six times the rate of methyl sterol demethylation. The very significant competition between esterification and demethylation of methyl sterol intermediates of skin suggests that sterol intermediates accumulate in rat skin because of the rapid formation of esters that may not be further metabolized.

The formation and reduction of the 14,15-double bond in cholesterol biosynthesis

It was shown that 100mug quantities of 4,4'-dimethyl[2-(3)H(2)]cholesta-8,14-dien-3beta-ol (IIIa), tritiated cholesta-8,14-dien-3beta-ol, 4,4'-dimethyl[2-(3)H(2)]cholesta-7,14-dien-3beta-ol, dihydro[2-(3)H(2)]lanosterol and [24-(3)H]lanosterol were converted by a 10000g supernatant of rat liver homogenate into cholesterol in 17%, 54%, 6%, 9.5% and 24% yields respectively. From an incubation of dihydro[3alpha-(3)H]lanosterol with a rat liver homogenate in the presence of a trap up to 38% of the radioactivity was found to be associated with a fraction that was unambiguously shown to be 4,4'-dimethylcholesta-8,14-dien-3beta-ol. Another related compound, 4,4'-dimethylcholesta-7,14-dien-3beta-ol was also shown to be equally effective in its ability to trap compound (IIIa) from an incubation of dihydro[3alpha-(3)H]lanosterol. The mechanism of the further conversion of the compound (IIIa) into cholesterol occurred by the reduction of the 14,15-double bond and involved the addition of a hydrogen atom from the medium to C-15 and another from the 4-position of NADPH to C-14. Two possible mechanisms for the removal of the 14alpha-methyl group in sterol biosynthesis are discussed.

Desmosterol as the major sterol in L-cell mouse fibroblasts grown in sterol-free culture medium

The principal sterol synthesized by L-cell mouse fibroblasts is desmosterol. Cholesterol was not detected in these cells when they were grown in a sterol-free culture medium. These findings indicate that, in cells, cholesterol can be replaced by desmosterol. Sterol analyses of six other tissue culture cell lines revealed cholesterol synthesis.

The stereochemistry of hydrogen elimination during 7,8-double bond formation by Tetrahymena pyriformis

It is shown that formation of the 7,8-double bond in the conversion of cholesterol into cholesta-5,7,22-trien-3beta-ol involves the removal of the 7beta- and 8beta-hydrogen atoms.

Assay of the possible organization of particle-bound enzymes with squalene synthetase and squalene oxidocyclase systems

Lanosterol was biosynthesized in pig liver homogenate from [4,8,12-(14)C(3)]farnesyl pyrophosphate and [4S-4-(3)H]NADPH through the intermediary formation of squalene labelled asymmetrically with (3)H. The biosynthetic lanosterol, freed from labelled 24,25-dihydrolanosterol, which was also synthesized, was converted into 24,25-dihydrolanosteryl acetate and subjected to chemical degradations to locate the position(s) of the (3)H label in the molecule. The ratio of (3)H at C-11 to that at C-12 was found to be 1.28. Although a certain inequality of labelling was thus indicated, experimental uncertainties did not permit the conclusion that the asymmetrically labelled squalene might have been cyclized preferentially from one end.