Xenobiotic Metabolism

James & Elizabeth Miller

Julius Axelrod

In Vitro Technology

Early studies of drug metabolism verified the central role of the liver. As the only organ positioned between the intestines and systemic blood circulation, this was the logical site for chemical transformation. In vitro techniques utilizing liver tissue became feasible once Potter and Elvehjem developed non-destructive homogenization procedures in 1936. Individual liver organelles could be isolated once A. Claude introduced differential centrifugation in 1941. Gerald Mueller, working in the laboratory of James and Elizabeth Miller, applied these techniques to the study of a rat carcinogen called DAB (dimethylaminoazobenzene). Mueller and Miller showed that DAB was metabolized by rat liver homogenates, and that the process required both oxygen and a reductant (NADPH).

The importance of this enzyme system was uncovered by Julius Axelrod, who turned down a career at the post office in the late 1930's for a lower-paying lab job at New York University. While there, he was asked to determine why some people were turning blue after taking the popular pain remedy Bromo Seltzer. He consulted with Bernard Brodie at NIH, and they found that the problem involved the metabolic conversion of the ingredient acetanilide to aniline, which induced the condition methemoglobinemia. The other metabolic product they discovered was acetaminophen, which was both safer and a better analgesic. Axelrod then identified the enzymes in the liver responsible for this process and was so intrigued that he went back to school to obtain M.S. and Ph.D. degrees.

At the age of 35, Axelrod moved to NIH and began studies on sympathomimetic amines, including amphetamine. He wondered why rabbits could rapidly metabolize this compound and what the mechanism might be? When he voiced concerns about his own lack of formal training in enzymology, his co-worker Gordon Tomkins replied "Julie, there's no big mystery to being an enzymologist. All you have to have is a razor blade and a liver." Axelrod found that amphetamine was also rapidly metabolized in liver slices, and learned that NADPH was required as a co-factor. Then he located the sub-cellular site of activity using the methods developed by Hogeboom and Schneider. This newly defined "microsomal" oxidizing system was soon shown to be responsible for metabolism of a wide variety of drugs and other chemicals.

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