The prospective role of abnormal methyl metabolism in cadmium toxicity.

Several lines of evidence point to the probable role of abnormal methylation processes in the toxicology of metals and other xenobiotics. The spectrum of toxic effects exhibited by such metals as Ni, As, and Cd, as well as by Zn deficiency, often resemble those seen in animals chronically fed methyl-deficient diets. These metal-associated pathologies include cancer, atherosclerosis, birth defects, neurological disturbances, and pancreatic lesions. In addition, each of the above agents has been shown to alter normal methyl group metabolism in vivo or in vitro. In the present studies, we compared the effects on the enzyme DNA methyltransferase (MTase) of two metal ions: the essential metal Zn and the carcinogen Cd. MTase extracts were obtained from the hepatic nuclei of rats fed a methyl-deficient diet (lacking choline and folate) for 7 and 24 weeks. Control animals were fed the same diet supplemented with each of these vitamins. Zn and Cd both inhibited MTase in the nuclear extracts from both the control and the methyl-deficient rats. The inhibitory activity of Cd was greater than that of Zn regardless of whether the nuclear extracts were from the control or the deficient animals. In addition, the kinetics of Cd inhibition of MTase activity were different in the nuclear extracts from the control and methyl-deficient rats. The results provide evidence that the carcinogenic effects of Cd may be mediated in part through abnormal DNA methylation.

Biotin deficiency reduces expression of SLC19A3, a potential biotin transporter, in leukocytes from human blood.

In evaluating potential indicators of biotin status, we quantitated the expression of biotin-related genes in leukocytes from human blood of normal subjects before and after inducing marginal biotin deficiency. Biotin deficiency was induced experimentally by feeding an egg-white diet for 28 d. Gene expression was quantitated for the following biotin-related proteins: methylcrotonyl-CoA carboxylase chains A (MCCA) and B (MCCB); propionyl-CoA carboxylase chains A (PCCA) and B (PCCB); pyruvate carboxylase (PC); acetyl-CoA carboxylase isoforms A (ACCA) and B (ACCB); holocarboxylase synthetase (HCS); biotinidase; and 2 potential biotin transporters: sodium-dependent multivitamin transporter (SMVT) and solute carrier family 19 member 3 (SLC19A3). For 7 subjects who successfully completed the study, the abundance of the specific mRNAs was determined by quantitative real-time RT-PCR at d 0 and 28. At d 28, SLC19A3 expression had decreased to 33% of d 0 (P < 0.02 by two-tailed, paired t test). Expression of MCCA, PCCA, PC, ACCA, ACCB, HCS, biotinidase, and SMVT decreased to approximately 80% of d 0 (P < 0.05). Expression of the MCCB and PCCB chains that do not carry the biotin-binding motif did not change significantly; we speculate that expression of the biotin-binding chains of biotin-dependent carboxylases is more responsive to biotin status changes. These data provide evidence that expression of SLC19A3 is a relatively sensitive indicator of marginal biotin deficiency.