Age- and tumour-related changes in methionine biosynthesis in mice

Methylation of estrogen receptor alpha and mutL homolog 1 in normal colonic mucosa: association with folate and vitamin B-12 status in subjects with and without colorectal neoplasia

BACKGROUND

Greater promoter methylation in some tumor-suppressor genes underlies most sporadic colorectal cancers and increases with age in the colon.

OBJECTIVE

We tested the hypothesis that biomarkers of folate and vitamin B-12 status are associated with estrogen receptor alpha (ERalpha) and mutL homolog 1 (MLH1) promoter methylation in subjects with and without neoplasia.

DESIGN

Biopsies of normal-appearing colorectal mucosa from 156 subjects with and without colorectal neoplasia (disease free, n = 76; cancer, n = 28; adenoma, n = 35; hyperplastic polyps, n = 17) were obtained at colonoscopy and used to evaluate methylation in 7 CpG sites in the ERalpha promoter and 13 CpG sites in the MLH1 promoter. Blood samples were obtained for the measurement of serum and red cell folate, serum vitamin B-12, and plasma homocysteine concentrations. Methylation indexes were generated to reflect an average methylation value across all CpG dinucleotides in both ERalpha and MLH1.

RESULTS

The methylation indexes for ERalpha and MLH1 generally were significantly (P < 0.05) higher in subjects with neoplasia than in disease-free subjects. The ERalpha methylation index correlated negatively with serum vitamin B-12 (r = -0.239, P = 0.003) and positively with plasma homocysteine (r = 0.188, P = 0.021). Disease status (P < 0.005), age (P < 0.001), and serum vitamin B-12 concentrations (P = 0.006) were independent determinants of ERalpha promoter methylation. Serum and red cell folate concentrations had no influence on ERalpha promoter methylation.

CONCLUSION

Serum vitamin B-12 but not folate status may be associated with ERalpha promoter methylation in normal-appearing colorectal mucosa.

Tumour effect on arginine/ornithine metabolic relationship in hypertrophic mouse kidney

The presence of a tumour significantly changes nitrogen metabolism, including that of amino acids and polyamines, in host animals. In this study, we examine whether developing tumours affect the metabolic relationship of arginine and ornithine, precursors of polyamines, in the testosterone-induced hypertrophic mouse kidney model. Androgen-induced changes in the activity of enzymes involved with ornithine biosynthesis (arginase), its consumption (ornithine aminotransferase, OAT and ornithine decarboxylase, ODC) and the hypertrophy of host mouse kidney were not affected by the presence of an ascitic tumour (EAC) and only slightly by a mammary carcinoma (MaCa). The HPLC determined renal level of arginine and ornithine showed a striking homeostasis and was disturbed neither by testosterone nor EAC. The effect of MaCa and testosterone on the levels of both amino acids, although significant, was not very pronounced. Developing tumours, especially ascitic, altered the renal activity of OAT and ODC, but not of arginase, in testosterone-untreated mice. All examined tumours, EAC, L 1210 and MaCa actively metabolized arginine and ornithine. the tumour content of arginine which coincided with the activity of arginase, resulted in a marked increase of the ornithine/arginine ratio in tumours, when compared with kidneys. These results indicate that the androgen-induced anabolic response in mouse kidney is preserved, in spite of tumour requirements for essential metabolites.

Tissue-specific effects of testosterone on S-adenosylmethionine formation and utilization in the mouse

Exogenous administration of testosterone produced several metabolic tissue-specific changes in female mouse kidneys, but not in the liver. The hormone induced ornithine decarboxylase (ODC) activity, and also profoundly influenced metabolism of S-adenosylmethionine (AdoMet). Therefore, the activity of the AdoMet-synthesizing enzyme (AdoMet synthetase) and of cystathionine synthase, which commits homocysteine irreversibly to the transsulfuration pathway, were significantly increased. In contrast to the level of AdoMet in the liver the renal level of this metabolite was augmented, whereas the level of S-adenosylhomocysteine (AdoHcy) did not change. This resulted in an increase of the AdoMet/AdoHcy ratio. In testosterone-treated mice, pulse-labelled with [methyl-14C]methionine, the radioactivity recovered in the kidneys doubled, but in the liver remained the same. The rise in radioactivity recovered occurred mainly in TCA-soluble compounds and lipids, and to a smaller extent, in proteins and nucleic acids.

Betaine:Homocysteine methyltransferase from rat liver: Purification and inhibition by a boronic acid substrate analog

Betaine:homocysteine methyltransferase (BHMT) from rat liver has been highly purified by an efficient procedure requiring only two chromatographic steps: Sephadex G-100 chromatography and fast protein liquid chromatography chromatofocusing. A 170-fold purification and 7.5% overall yield were achieved. Chromatofocusing yielded three active forms of BHMT with pI values near 8.0, 7.6, and 7.0. The subunit molecular weight of each active form is 45,000 Da as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the native enzyme has a molecular weight of 270,000 as determined by exclusion chromatography. The stability of the purified enzyme was found to be potentiated by the presence of 1 mM dimethylglycine and 1 mM homocysteine. Boronate analogs of betaine (pinanyl N,N,N-trimethylaminomethaneboronate) (4) and dimethylglycine (pinanyl N,N-dimethylaminomethaneboronate) were synthesized from pinanyl iodomethaneboronate (3) and trimethylamine or dimethylamine, respectively. The free acid of the betaine analog (5) was reversibly generated from (4). The inhibition of BHMT by (5) appears competitive with a Ki = 45 microM. Since the Km for betaine measured with the purified enzyme is near 0.1 mM, the boronic acid analog of betaine appears to function effectively as a substrate analog inhibitor of BHMT. The analog does not appear to act as a methyl donor to homocysteine when (5) is substituted for betaine in the enzyme reaction. In addition, an enzyme assay based upon C3-cyano reverse phase HPLC detection of the o-phthalaldehyde derivative of methionine was developed as an alternative to the standard radiochemical assay. Betaine:homocysteine methyltransferase in the picomole range can be quantitated using this assay as indicated by a linear response of enzyme activity to protein concentration.