Quantitative determination of carcinogens and mutagens as alkylating agents following chemical activation

Evidence for metabolism of N-nitrosoproline

The metabolism of nitrosoproline (NPRO) was re-investigated in uni- and bilaterally nephrectomized rats that have reduced or absent ability to excrete urine. About 1% of the administered radioactivity from L-[U-14C]-NPRO appeared as 14CO2 in the expired air and the production of 14CO2 was time-dependent over a period of 23 h. As compared with sham-operation, uni- or bilateral nephrectomy did not significantly increase the amount of NPRO metabolism, though urinary excretion of radioactivity was decreased in the unilaterally nephrectomized animals. In microsome-mediated and in vitro enzyme-free (Udenfriend-hydroxylating) systems covalent binding of [2,3,4,5-3H]NPRO to exogenous calf thymus DNA was demonstrated. The above findings confirm that in vivo metabolism of NPRO is possible, albeit, to a very small extent.

Metabolic studies of 15N-labeled N-nitrosoproline in isolated rat hepatocytes and subcellular fractions

The metabolism of the non-carcinogenic N-nitrosoproline (NPRO) was investigated in vitro using both S9 preparations and isolated hepatocytes from F344 rats. The studies were performed using 15N-labeled nitrosamine and the reaction mixtures were examined mass spectrometrically for the presence of 15N2 or other 15N-labeled gaseous products. In addition, the metabolism of NPRO was monitored by capillary gas chromatography. The results indicated no 15N2 production from either the hepatocyte or S9 preparations, as well as no detectable loss of substrate from the reaction mixtures. Mass spectrometric analysis failed to reveal any metabolites of NPRO. The results suggest that NPRO may be refractory to the normal nitrosamine activating enzymes, confirming its suitability for use in human epidemiological studies of endogenous nitrosation.

Alkylation of guanosine and 4-(p-nitrobenzyl)-pyridine by styrene oxide analogues in vitro

In vitro alkylation activity of styrene oxide (SO) and three of its analogues, 4-vinyltoluene oxide (VTO), 3,5-dimethylstyrene oxide (DMSO), and 4-nitrostyrene oxide (NSO) was assayed using 4-(p-nitrobenzyl)pyridine (NBP) and guanosine as nucleophiles. Hydrolysis rates were also determined. The half-lives of VTO, DMSO, SO, and NSO were 8, 11, 40, and 60 h, respectively in aqueous solution. The rate of NBP alkylation correlated with the rate of hydrolysis. By contrast, the rates of guanosine alkylation were quite different: SO greater than VTO greater than DMSO. NSO did not react with guanosine. Fluorescence an ultraviolet spectroscopic data on the guanosine adducts indicated that SO, VTO, and DMSO formed main alkyl products at N-7.