Search PubMed⌕ Search

Biomedical subjects

S K Shapiro

Publications and source records attributed to S K Shapiro.

At least 37 records · Page 2Linked to original sources
PubMed1977

Function of S-adenosylmethionine in germinating yeast ascospores.

Germination and outgrowth of ascospores of Saccharomyces cerevisiae 4579 require both methionine and adenine, whereas leucine is only required for outgrowth. The methionine requirement may be satisfied by S-adenosylmethionine, but this sulfonium compound will not substitute for adenine. Between 30 and 70 min of protein synthesis is initially required for the completion of germination in strain 4579. The inhibition of S-adenosylmethionine synthetase by trifluoromethionine prevents both germination and protein synthesis. During the initial stages of germination, the S-adenosylmethionine synthetase, S-adenosylmethionine decarboxylase, and transfer ribonucleic acid methyltransferases increased significantly, indicating that polyamines and/or the methylation of transfer ribonucleic acid are required for the initiation of germination.

Adenine↗
PubMed1976

Kinetic properties and the effect of substrate analogues on 5'-methylthioadenosine nucleosidase from Escherichia coli.

5'-Methylthioadenosine nucleosidase (EC 3.2.2-) from Escherichia coli has been purified 220-fold. A molecular weight of 31 000 for the enzyme was estimated from gel filtration on Sephadex G-150. The Km for 5'-methylthioadenosine was 3.1-10(-7) M. In addition to 5'-methylthioadenosine, the nucleoside analogues 5'-ethylthioadenosine, 5'-n-propylthioadenosine, and S-adenosyl-homocysteine also served as substrates for the enzyme. These substrate analogues acted as competitive inhibitors of the reaction with 5'-methylthioadenosine. The Ki values for 5'-ethylthioadenosine, 5'-n-propylthioadenosine, and S-adenosylhomocysteine were determined to be 1.3-10(-7) M, 4.6-10(-8) M, and 1.92-10(-7) M respectively.

Adenosine↗
PubMed1975

Assay and regulation of S-adenosylmethionine synthetase in Saccharomyces cerevisiae and Candida utilis.

A simple and sensitive assay for S-adenosylmethionine (SAM) synthetase is described which depends on the quantitative separation of the product, [14CH3]S-adenosylmethionine, from the substrate, L-[14CH3]methionine, on a Bio-Rex 70 column. L-Methionine protects the enzyme during preparation of cell extracts by sonic treatment but causes repression of enzyme activity during growth of Candida utilis. The presence of 5 mM methionine in the growth medium repressed SAM synthetase specific activity threefold compared to the specific acitivity of the enzyme isolated from cells grown in unsupplemented medium. Conversely, the presence of methionine in the growth medium resulted in an 80-fold increase in the intracellular concentration of SAM as compared to the Sam accumulated intracellularly in unsupplemented cultures.

Adenosine Triphosphate↗
PubMed1972

Sai-1 mutation: saccharomyces cerevisiae: characteristics of inhibition by S-adenosylmethonine and S-adenosylhomocysteine and protection by methionine.

The relationship of methionine to the inhibition caused by S-adenosylmethionine and S-adenosylhomocysteine in strains containing the sai-1 mutation has been investigated and shown to affect indirectly the survival of the mutants. The ability of the mutants to take up both inhibitors is similar to that of the wild-type cells. The mutant also retains the ability to hydrolyze S-adenosylhomocysteine and incorporate the hydrolytic products into the various cellular fractions. Maximal inhibition of the sai-1 mutants occurs at an extracellular concentration of 0.005 mmS-adenosylmethionine and 0.025 to 0.05 mmS-adenosylhomocysteine when the cellular concentration is 0.05 mg (dry weight) per ml. The results suggest that the sai-1 mutation affects reaction(s) either not associated with methionine biosynthesis, or methionine synthesis and at least one other critical cellular function.

Adenine↗
PubMed1967

Some mutants of Saccharomyces cerevisiae inhibited by adenoylmethionine and adenosylhomocysteine.

These investigations have established the existence of a novel type of non-nutritional mutant (ai) which is inhibited in the presence of two naturally occurring cellular compounds. The inhibition is complete at an extracellular concentration at least as low as 0.05 mumole/ml of either adenosylhomocysteine or adenosylmethionine. It is suggested that adenosylhomocysteine is the true inhibitor. The ai mutants are phenotypically indistinguishable from the wild type in the absence of inhibitors. The results have shown that, if any direct effect on the methionine biosynthetic pathway exists, it is a secondary rather than the primary effect of the inhibitors. The ai mutation does not involve the loss of the adenosylmethionine (or methylmethionine): homocysteine methyltransferase. In addition, the ai mutants accumulate, maintain, and utilize adenosylmethionine and methionine in a manner similar to the parental strain. No genetic relationship could be detected between the ai-1 mutation and several different markers affecting methionine biosynthesis. The ai-1 mutation was also shown to be genetically recessive. Methionine partially reverses the inhibition caused by adenosylmethionine or adenosylhomocysteine. Neither methylmethionine nor homocysteine reversed the inhibition, which showed that the homocysteine methyltransferase cannot catalyze the synthesis of sufficient methionine under these conditions to simulate the effects of extracellularly supplied methionine. If adenine is present, methionine does not cause reversal of inhibition due to adenosylmethionine or adenosylhomocysteine. From the data presented, it is clear that the ai mutation involves some metabolic control mechanism, though the alteration does not appear to be associated primarily with the biosynthesis of methionine.

Adenine↗
PubMed1967

Dominant mutation for ethionine resistance in Saccharomyces cerevisae.

An ethionine-resistant mutant of Saccharomyces cerevisiae has been investigated whose mutation (et(r2)) confers resistance to the heterozygous diploid also containing the sensitive allele, et(s). The mutation is apparently specific for reversal of ethionine inhibition. The principal difference between the sensitive et(s) strain and the mutant was the latter's inability to concentrate large intracellular quantities of adenosylethionine. Reduced incorporation of ethyl groups or ethionine in other cellular fractions of the mutant was also detected. The data show that the mutant has not lost the ability to form adenosylethionine. It is suggested that the mutant has an increased ability to hydrolyze this sulfonium compound after it has been synthesized. It is possible that some of the ethionine is detoxified before it can participate in protein or adenosylethionine synthesis. No mutant alteration in accumulation of ethionine from the medium was detected. In the presence of ethionine, the parental strain accumulated 25 times more adenosylethionine than did the mutant. However, with methionine, only twice as much adenosylmethionine was accumulated by the parental strain as by the mutant.

Carbon Isotopes↗