Methionine metabolism in pyridoxine deficiency

Methionine metabolism was studied in a group of subjects having clinical signs and symptoms of peripheral neuropathy. Following a load of methionine, these subjects excreted higher amounts of cystathionine, and the ratio of cystathionine/ cysteine sulfinic acid was elevated suggesting a block in the conversion of cystathionine to cysteine sulfinic acid. The load return of methionine as cystathionine was significantly more. These results indicate that the activity of enzyme cystathionase, a pyridoxal-dependant enzyme, is low in pyridoxine deficiency. With pyridoxine therapy, there was a significant improvement in the clinical status, and the biochemical abnormalities were corrected.

[Role of sulfated amino acids in the growth of Corynebacterium sepedonicum (Spieck, et Kott) Skapt. et Burkh]

Corynebacterium sepedonicum requires organic sulfur for its growth. In potato host tissue, the normal source of sulfur for the bacterium is methionine. The l isomer is more readily metabolized than the d isomer or the dl racemic mixture, but four derivatives, methionine sulfoxide, DL-methionine-CH3-sulfonium Cl, acetyl- and alanyl-methionine are more efficiently metabolized than any of the methionine isomers. Homocysteine and cystathionine, intermediates in methionine metabolism, are rapidly metabolized in the presence of trace amounts of methionine. Another intermediate, S-adenosylmethionine, has no effect on the bacterial growth. Ethionine and α-methylmethionine are not metabolized by C. sepedonicum and cystathionine or homocysteine cannot overcome the inhibitory effect of α-methylmethionine on growth of the organism.

In vivo synthesis of sulfur containing amino acids in Candida albicans

Growth in a supplemented minimal culture medium and chlamydospore formation in supplemented corn meal agar were the criteria used to evaluate in vivo biosynthesis of sulfur containing amino acids in auxotrophs of Candida albicans. Supplementation with L-cystathionine, L-homocysteine, L-methionine, S-adenosyl-L-methionine, or S-adenosyl-L-homocysteine resulted in growth in the minimal medium and both growth and chlamydospore formation in corn meal agar. One auxotroph (CYS) gave comparable results when inoculated into media supplemented with L-cysteine, responded minimally when O-acetyl-L-serine and sulfide comprised a dual supplement, and failed to grow or form chlamydospores when sulfide or O-acetyl-L-serine was used as a single supplement, suggesting that this organism may possess a deficiency in the synthesis or activity or cysteine synthetase. Another auxotroph (OAHS) did not grow or produce chlamydospores when O-acetyl-L-serine was added to the media in combination with sulfide, or when either L-cysteine, O-acetyl-L-serine, sulfide, or L-homoserine was used as a single supplement, but did respond to media supplemented with O-acetyl-L-homoserine indicating a deficiency in homoserine-O-transacetylase in this microorganism. Auxotroph OAHS grew and formed chlamydospores as well in media containing only supplemental O-acetyl-L-homoserine as it did when O-acetyl-L-homoserine was used in combination with sulfide S-methyl-L-cysteine or thiomethyladenosine, while (CYS) neither grew nor produced chlamydospores in any of these supplemented media. These results lend support for the hypothesis that cystathionine is an important intermediate in methionine biosynthesis in C. albicans. The positive results obtained when the auxotrophs were inoculated into media supplemented with S-adenosyl-L-homocysteine or S-adenosyl-L-methionine may reflect an alternate route for methionine biosynthesis which functions conditionally in this microorganism. The potential significance for endogenous formation of sulfur containing amino acids from S-adenosylmethionine is therefore considered.

Factors affecting virulence of Shigella flexneri: defective methionine synthesis in an Escherichia coli-Shigella hybrid

An Escherichia coli-Shigella flexneri hybrid of intermediate virulence was studied to determine whether its shorter survival in host cells might be due to a metabolic defect. Investigation of its growth in minimal glucose medium showed that the hybrid, like its E. coli parent, had a longer lag phase and a slower growth rate than its virulent Shigella parent. Methionine was found to increase the growth rate of the hybrid. The Shigella parent of the hybrid can synthesize methionine normally, but the E. coli parent has a point mutation in its metA gene. Since it synthesizes enough methionine to grow slowly, it is postulated that the hybrid strain has a hybrid metA gene, that is, a gene composed partly of deoxyribonucleic acid (DNA) from E. coli, with the balance of the DNA from S. flexneri. Phage P1-mediated transduction, with the metA(-)E. coli parent as recipient and the Shigella parent as donor, yielded a few transductants that responded to methionine in the same way the hybrid did. Many more transductants of the hybrid type were produced when the hybrid strain was used as a donor. It is suggested that this poorly functioning gene acts synergistically with the hybrid strain's relaxed synthesis of ribonucleic acid to prevent its survival in the host.

Cystationine synthesis in yeast: an alternative pathway for homocysteine biosynthesis

Cystathionine synthesis from O-acetylhomoserine and cysteine has been demonstrated in yeast extracts for the first time. The activity is less than that of O-acetylhomoserine sulfhydrylase, but it is higher than that reported for homoserine O-transacetylase and therefore should not be growth limiting. Cystathionine synthase seems to share the regulatory properties of the sulfhydrylase, and both activities are missing from the methionine auxotroph Saccharomyces cerevisiae EY9, suggesting that both reactions are catalyzed by the same enzyme. However, cystathionine synthase activity was lost during purification of the sulfhydrylase, suggesting that the two reactions may be catalyzed by separate enzymes. Since previous studies have shown that yeast extracts can catalyze the cleavage of cystathionine to homocysteine, our results show the existence of two complete alternate pathways for homocysteine biosynthesis in yeast. Which of these is the major physiological pathway remains to be determined.

The formation of sulphur-containing amino acids in germinating seeds of rape (Brassica napus L.)

Sodium [(35)S]sulphide was fed to batches of germinating rapeseed, in some instances with the addition of unlabelled cysteine. Both the total radioactivity and specific radioactivity of the free sulphur-containing amino acids were examined. Cysteine and homocysteine were rapidly labelled; label subsequently appeared in cystathionine and methionine. The results obtained indicated that both the sulphydration and trans-sulphuration pathways were operating. This conclusion was reinforced by the results of experiments in which batches of rapeseed were incubated with l-[(14)C]homoserine. These showed the formation of labelled homocysteine, cystathione and methionine. It was thought the trans-sulphuration pathway was making the greater contribution to the biosynthesis of methionine in germinating rapeseed.