Metal-binding chelators may interact with biological systems by either of two mechanisms: they may combine with an essential metal, which can be either freely dissociated or part of an enzyme prosthetic group, or they may react with a metal ion to form a biologically reactive metal-chelate complex. As trace metals are always present as contaminants in serum-supplemented culture media used to study chelating agents, it is frequently difficult to distinguish between the two possibilities. Here we describe the use of a nontoxic, copper-specific chelating agent, bathocuproine sulphonate (Fig. 1) which, by combining with available endogenous copper in a tissue culture preparation, abolished the toxicity of three structurally unrelated chelating agents. These three agents may therefore be considered to be biologically active by the second mechanism.

Continued administration of several hepatocarcinogens led to an increase in the concentration of glutathione (GSH) in the livers of intact, but not of hypophysectomized or adrenalectomized rats. The concentration of GSH remained high untill the development of hyperplastic nodules. Subsequently, the concentration of GSH dropped to the normal level or below. A single dose of 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) produced an increase of GSH which, within a certain range, depended upon the amount of the carcinogen. In well differentiated, slowly growing hepatomas, the concentration of GSH approached the level in normal adult rat liver. On the other hand, in nondifferentiated and rapidly growing hepatomas, GSH was only 30-40% of that in normal liver. The activity of gamma-glutamyl transpeptidase (GTase) increased within 24-48 hours after a single large dose of 3'-Me-DAB. Continued feeding of 3'-Me-DAB led to an exponential increase of GTase. During hepatocarcinogenesis, the level of GTase activity corresponded to the degree and size of pathologic changes produced in rat liver. Chloramphenicol partially inhibited the increase of GTase induced by 2-acetylaminofluorene. Pretreatment with 3-methylcholanthrene partially inhibited the increase of GTase that had been induced by a single dose of 3'-Me-DAB. Puromycin partially inhibited the increase of GTase induced by several doses of dimethylnitrosamine. These observations indicated a close connection between the activation of GTase and chemical carcinogenesis in rat liver. Measurements of GTase activity in 12 Morris hepatomas supported this conclusion; their GTase levels were greatly elevated compared with that in normal adult rat liver.

No abstract