Covalent binding and endogenous incorporation as illustrated by nitroso carcinogens

Abstract

Serious problems of interpretation may arise when metabolic studies are carried out with radioactively labeled drugs and other foreign chemicals. Detection of free or bound radioactivity in tissues or body fluids may indicate the presence of the unchanged chemical or of various products of its decomposition. Some radioactivity may, however, represent incorporation of certain metabolites of foreign chemicals into body constituents by normal biosynthetic pathways. The interpretation of the results of such metabolic experiments from the standpoint of safety evaluation will be profoundly different if the radioactivity represents covalent binding to a cellular macromolecule than if it results from normal endogenous incorporation.These two distinct types of binding of radioactivity to body constituents are well illustrated by experimental studies with some carcinogenic nitroso compounds, such as dimethylnitrosamine and H-methylnitrosourea. Dimethylnitrosamine requires metabolic activation by microsomal enzymes and yields formaldehyde and a chemically reactive methylating intermediate, probably a methyl carbonium ion. The latter reacts with nucleophilic sites in nucleic acids and proteins, and also with water to yield methanol. H-Methylnitrosourea does not require metabolic activation but yields the same methylating intermediate spontaneously under physiologic conditions. Both formaldehyde and methanol are metabolized largely to CO 2 , but they also enter the one-carbon metabolic pool and became biosynthetically incorporated into nucleic acids, proteins, and other cell components. Alkylation of cellular constituents is associated with various biological effects, including cytotoxicity, carcinogensis, and mutagenesis, and the same effects are produced by the activated forms of a variety of other chemical carcinogens. It is clearly of paramount importance to distinguish between these two types of incorporation of radioactivity.

Studies on the Tissue-Disposition and Fate of [14c]streptozotocin With Special Reference to the Pancreatic Islets

“…The methyl group which is split off will however, also react with water to form methanol and formaldehyde (Magee 1977). These are either oxidized to carbon dioxide which is exhaled or are incorporated via normal metabolic pathways in the one-carbon pool of the body (Magee 1977). Therefore, if a cleavage of the labelled methyl group of the N-nitrosomethylurea side-chain of streptozotocin occurs, 14COo exhalation and incorporation of radioactivity in the macromolecules of the tissues may be expected.…”

“…N-nitrosomethylurea, which constitutes the side-chain of strepto¬ zotocin (Herr et al 1967), has been shown to methylate nucleic acids in several organs of rats (Swann 8c Magee 1968). The methyl group which is split off will however, also react with water to form methanol and formaldehyde (Magee 1977). These are either oxidized to carbon dioxide which is exhaled or are incorporated via normal metabolic pathways in the one-carbon pool of the body (Magee 1977).…”

A prospective on covalent binding and toxicity

“…The studies presented at this symposium by Magee (1977) illustrate that dimethylnitrosamine, a carcinogen present in many foods, is converted to formaldehyde that serves as a precursor of the methyl group of methionine. Moreover, studies by Rosenblum (p. 803) illustrated that at least a portion of some foreign compounds may ultimately be converted into threonine, an essential amino acid in rats.…”