No abstract

In mice depleted of GSH by treatment with buthionine sulfoximine (BSO), methimazole (2-mercapto-1-methylimidazole, MMI) causes liver injury characterized by centrilobular necrosis of hepatocytes and an increase in serum alanine transaminase (SALT) activity. MMI requires metabolic activation by both P450 monooxygenase and flavin-containing monooxygenase (FMO) before it produces the hepatotoxicity. MMI and its analogues were examined for the ability to increase SALT activity in GSH-depleted mice. Saturation of the C-4,5 double bond in MMI resulted in a complete loss of hepatotoxicity. Similarly, ring fusion of a benzene nucleus to the C-4,5 double bond, forming 2-mercapto-1-methylbenzimidazole, abolished the toxic potency. As for MMI, 2-mercapto-1,4,5-trimethylimidazole, and 2-mercapto-1-methyl-4, 5-di-n-propylimidazole, the toxic potency decreased with the increasing bulk of the 4- and 5-alkyl substituents. Furthermore, methylation of the thiol group of MMI totally reduced its toxicity. These structural requirements and the known toxicity of thiono-sulfur compounds led us to the hypothesis that MMI would undergo epoxidation of the C-4,5 double bond by P450 enzymes and, after being hydrolyzed, the resulting epoxide would be then decomposed to form N-methylthiourea, a proximate toxicant. Before N-methylthiourea would produce toxicity, it would be further biotransformed to its S-oxidized metabolites mainly by FMO. Evidence for this hypothesis was provided by the facts that N-methylthiourea and glyoxal as the accompanying fragment were identified as urinary metabolites in mice treated with MMI and that N-methylthiourea caused a marked increase in SALT activity when administered to mice in combination with BSO.

Methimazole (MMI) (Ͼ0.1 mmol kg ؊1 , p.o.) given in combination with DL-buthionine sulphoximine (BSO) (3 mmol kg ؊1 , i.p., 1 h before MMI administration), an inhibitor of glutathione (GSH) synthesis, caused liver injury in mice. The injury was characterized by centrilobular necrosis of hepatocytes and an increase in serum alanine transaminase (ALT) activity. Methionazole (2 mmol kg ؊1 ) alone resulted in only a marginal increase in serum ALT activity, but produced no histopathological changes in the liver. Pretreatment with hepatic cytochrome P-450 monooxygenase inhibitors-cobalt chloride, isosafrole, methoxsalen, metyrapone and piperonyl butoxide-prevented or tended to suppress the hepatotoxicity induced by MMI in combination with BSO. Treatment with N,N-dimethylaniline and ethyl methyl sulphide, competitive substrates of flavin-containing monooxygenases (FMO), also resulted in remarkable suppression of the hepatotoxicity caused by MMI in combination with BSO. These results suggest that MMI is activated by reactions mediated by both cytochrome P-450 monooxygenases and FMO, and that the inadequate rates of detoxification of the resulting metabolite are responsible for the hepatotoxicity in GSH-depleted mice. Figure 1. The structure of methimazole.

Methimazole (MMI) (>0.1 mmol kg−1, p.o.) given in combination withDL‐buthionine sulphoximine (BSO) (3 mmol kg−1, i.p., 1 h before MMI administration), an inhibitor of glutathione (GSH) synthesis, caused liver injury in mice. The injury was characterized by centrilobular necrosis of hepatocytes and an increase in serum alanine transaminase (ALT) activity. Methionazole (2 mmol kg−1) alone resulted in only a marginal increase in serum ALT activity, but produced no histopathological changes in the liver. Pretreatment with hepatic cytochrome P‐450 monooxygenase inhibitors—cobalt chloride, isosafrole, methoxsalen, metyrapone and piperonyl butoxide—prevented or tended to suppress the hepatotoxicity induced by MMI in combination with BSO. Treatment with N,N‐dimethylaniline and ethyl methyl sulphide, competitive substrates of flavin‐containing monooxygenases (FMO), also resulted in remarkable suppression of the hepatotoxicity caused by MMI in combination with BSO. These results suggest that MMI is activated by reactions mediated by both cytochrome P‐450 monooxygenases and FMO, and that the inadequate rates of detoxification of the resulting metabolite are responsible for the hepatotoxicity in GSH‐depleted mice. Copyright © 1999 John Wiley & Sons, Ltd.

In four rice (Oryza sativa L.) mutants resistant to hydroxy-L-proline (Hyp), HYP101, HYP203, HYP205 and HYP210, and in their original variety, Nipponbare, free proline and Hyp contents in the seeds and in the 14-day-old seedlings have been determined. The four mutants can be divided into two groups: HYP101 and HYP203 are classified as to recessive gene and the levels of free proline are similar to that of the original variety; the second group includes mutants HYP205 and HYP210 where the Hyp resistance is transmitted heterozygously and, both in the seeds and in the seedlings, a remarkable increase in free proline content is observed. In particular, free proline contents in the seeds of HYP205 and HYP210 are, respectively, 24 and 12 times that of the original variety. Hyp is detected only in the seedlings cultured with Hyp solution. In the Hyp resistant seedlings of HYP205 and HYP210, Hyp contents are twice that of the original variety and less than half in the seedlings of HYP101 and HYP203. Hyp resistance and differential proline levels are also evident in the callus initiated from the mutants. This suggests that the Hyp resistant mutants are good genetic markers both in planta and in vitro. The Hyp mutants are also discussed with regard to stress resistance.