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Biomedical subjects

M Akhtar

Publications and source records attributed to M Akhtar.

At least 523 records · Page 29Linked to original sources
PubMed1971

Studies on the mechanism and regulation of C-4 demethylation in cholesterol biosynthesis. The role of adenosine 3':5'-cyclic monophosphate.

1. An assay for demethylation has been developed based on the release of tritium from 4,4-dimethyl[3alpha-(3)H]cholest-7-en-3beta-ol (II). 2. The maximum release of (3)H from 3alpha-(3)H-labelled compound (II) in a rat liver microsomal preparation occurs in the presence of NADPH and NAD(+) under aerobic conditions. 3. Incubation of 3alpha-(3)H-labelled compound (II) with NADPH under aerobic conditions leads to the formation of a 3alpha-(3)H-labelled C-4 carboxylic acid. This compound undergoes dehydrogenation on subsequent anaerobic incubation with NAD(+). 4. The (3)H released from the steroid was located in [4-(3)H]nicotinamide and the medium. Incubation with synthetic [4-(3)H(2)]NADH gave a similar result. 5. In the presence of glutamate dehydrogenase and alpha-oxoglutarate part of the (3)H released from the steroid was transferred to glutamate. 6. A series of 3-oxo steroids were reduced equally well by [4-(3)H(2)]NADH and [4-(3)H(2)]NADPH. The reduction of 5alpha-cholest-7-en-3-one was shown to use the 4B H atom from the nucleotide. 7. 3':5'-Cyclic AMP was shown to be a competitive inhibitor of the 3beta-hydroxy dehydrogenase enzyme in the demethylation reaction.

Aerobiosis↗
PubMed1970

The mechanism of action of serine transhydroxymethylase.

1. The preparation of stereospecifically tritiated glycines and the determination of their absolute configurations by the use of d-amino acid oxidase are described. 2. The reaction catalysed by serine transhydroxymethylase, which results in the conversion of glycine into serine, has been separated into at least four partial reactions. It is suggested that the first event in this conversion is the formation of a Schiff base intermediate of glycine and pyridoxal phosphate. The next important step involves the removal of the 2S-hydrogen atom of glycine to give a carbanion intermediate. Experiments pertinent to the mechanism of conversion of this carbanion intermediate into serine are described. 3. The enzyme preparation catalysing the conversion of glycine into serine also participates in the conversion of glycine into threonine and allothreonine. In both these conversions, glycine --> serine and glycine --> threonine, the 2S-hydrogen atom of glycine is eliminated and the 2R-hydrogen atom of glycine is retained. 4. In the light of these experiments the mechanism of action of serine transhydroxymethylase is discussed. It is suggested that methylenetetrahydrofolate is the carrier of formaldehyde, from which formaldehyde may be liberated at the active site of the enzyme, thus allowing the overall reaction to take place.

Folic Acid↗
PubMed1970

The stereochemistry of hydrogen elimination from C-7 in cholesterol and ergosterol biosynthesis.

The synthesis of [7alpha-(3)H]lanosterol is described. It is shown that in the conversion of [7alpha-(3)H,26,27-(14)C(2)]lanosterol into cholesterol by a rat liver system, it is the 7beta-hydrogen atom that is predominantly removed. On the other hand, the conversion of doubly labelled lanosterol into ergosterol by whole yeast cells results in the loss of the 7alpha-hydrogen atom. These results therefore suggest that the C-7 hydrogen atoms with opposite stereochemistry are labilized by the rat liver and the yeast Delta(8)-Delta(7) steroid isomerases.

Animals↗
PubMed1970

A convenient synthesis of labelled rhodopsin and studies on its active site.

Digitonin solutions of labelled rhodopsin, containing (3)H in the retinyl moiety, were prepared by two related methods. Labelled rhodopsin was also prepared for the first time in cetyltrimethylammonium bromide and purified by column chromatography. It was shown that only certain rhodopsin preparations on denaturation in the dark and the reduction with sodium borohydride gave up to 60% of the radioactivity in a fraction characterized as N-retinylphosphatidylethanolamine. Such preparations also gave a lipid-linked retinyl moiety at the metarhodopsin-I stage, but, as expected, a protein-linked retinyl moiety at the metarhodopsin-II stage. Other preparations however, gave exclusively protein-bound radioactivity at the native-rhodopsin, metarhodopsin-I and metarhodopsin-II stages. It is therefore conceivable that the formation of N-retinylphosphatidylethanolamine is due to a non-enzymic reaction resulting from the transfer of the retinyl moiety from its native site to an amino group of a favourably oriented phospholipid molecule. The only firmly established aspect of the rhodopsin active site remains the demonstration in our previous work that at the metarhodopsin-II stage the retinyl moiety is linked to an in-amino group of lysine. On the basis of chemical reactivity it is argued that the light-induced conversion of rhodopsin into metarhodopsin II involves a profound conformational change resulting in the dislocation of the retinylideneiminium chromophore from a non-polar environment in rhodopsin to a polar environment in metarhodopsin II.

Amines↗
PubMed1969

The role of a cholesta-8,14-dien-3-beta-ol system in cholesterol biosynthesis.

The biosynthesis of cholesterol from squalene and tritiated water is described. Degradation of the cholesterol indicated that C-15 may be involved in cholesterol biosynthesis. In accordance with this view it is shown that in the conversion of [2RS-(3)H(2)]mevalonic acid into cholesterol one of the hydrogen atoms at C-15 is removed. A mechanism for the removal of the 14alpha-methyl group in steroid biosynthesis that involves the labilization of a C-15 hydrogen atom is outlined. In accordance with the requirement of this scheme it is shown that 4,4'-dimethyl-cholesta-8,14-dien-3beta-ol is converted into cholesterol.

Animals↗
PubMed1969

Studies on the biosynthesis of the ergosterol side chain.

1. A convenient synthesis of 24-methylene[23,25-(3)H(3)]dihydrolanosterol is described. 2. A general anaerobic-aerobic method for the incorporation of sterols into whole yeast cells is also described and illustrated by experiments with (3)H-labelled lanosterol. 3. The method was used to convert labelled 24-methylene-dihydrolanosterol into ergosterol, in good yield, by Saccharomyces cerevisiae. 4. Degradation of the biosynthetic ergosterol provided confirmation of the conversion, which supports the proposed mechanism for the biosynthesis of the ergosterol side chain. 5. Mechanisms for the further conversion of the 24-methylene side chain into the ergosterol side chain are discussed and it was shown that a compound, [3alpha-(3)H(1)]-ergost-7-en-3beta-ol, with a fully saturated side chain, can also be efficiently incorporated into ergosterol. 6. This result was confirmed by a procedure involving formation of the 5,8-epidioxide and subsequently the 5,8-epidioxy-22,23-epoxide of the biosynthetic ergosterol.

Ethers, Cyclic↗
PubMed1969

The stereospecific removal of a C-19 hydrogen atom in oestrogen biosynthesis.

1. The synthesis of a number of 19-substituted androgens is described. 2. A method for the partially stereospecific introduction of a tritium label at C-19 in 19-hydroxyandrost-5-ene-3beta,17beta-diol was developed. The 19-(3)H-labelled triol produced by reduction of 19-oxoandrost-5-ene-3beta,17beta-diol with tritiated sodium borohydride is tentatively formulated as 19-hydroxy[(19-R)-19-(3)H]androst-5-ene-3beta,17beta-diol and the 19-(3)H-labelled triol produced by reduction of 19-oxo[19-(3)H]-androst-5-ene-3beta,17beta-diol with sodium borohydride as 19-hydroxy[(19-S)-19-(3)H]-androst-5-ene-3beta,17beta-diol. 3. In the conversion of the (19-R)-19-(3)H-labelled compound into oestrogen by a microsomal preparation from human term placenta more radioactivity was liberated in formic acid (61.6%) than in water (38.4%). In a parallel experiment with the (19-S)-19-(3)H-labelled compound the order of radioactivity was reversed: formic acid (23.4%), water (76.2%). 4. These observations are interpreted in terms of the removal of the 19-S-hydrogen atom in the conversion of a 19-hydroxy androgen into a 19-oxo androgen during oestrogen biosynthesis. 5. It is suggested that the removal of C-19 in oestrogen biosynthesis occurs compulsorily at the oxidation state of a 19-aldehyde with the liberation of formic acid.

Androgens↗