Cytochrome P450 2A-catalyzed metabolic activation of structurally similar carcinogenic nitrosamines: N′-Nitrosonornicotine enantiomers, N-nitrosopiperidine, and N-nitrosopyrrolidine

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Abstract

N′-Nitrosonornicotine (NNN) and N-nitrosopiperidine (NPIP) are potent esophageal and nasal cavity carcinogens in rats and pulmonary carcinogens in mice. N-Nitrosopyrrolidine (NPYR) induces mainly liver tumors in rats and is a weak pulmonary carcinogen in mice. These nitrosamines may be causative agents in human cancer. α-Hydroxylation is believed to be the key activation pathway in their carcinogenesis. P450 2As are important enzymes of nitrosamine α-hydroxylation. Therefore, a structure-activity relationship study of rat P450 2A3, mouse P450 2A4 and 2A5, and human P450 2A6 and 2A13 was undertaken to compare the catalytic activities of these enzymes for α-hydroxylation of (R)-NNN, (S)-NNN, NPIP, and NPYR. Kinetic parameters differed significantly among the P450 2As although their amino acid sequence identities were 83% or greater. For NNN, α-hydroxylation can occur at the 2′- or 5′-carbon. P450 2As catalyzed 5′-hydroxylation of (R)- or (S)-NNN with Km values of 0.74-69 μM. All of the P450 2As except P450 2A6 catalyzed (R)-NNN 2′-hydroxylation with Km values of 0.73-66 μM. (S)-NNN 2′-hydroxylation was not observed. Although P450 2A4 and 2A5 differ by only 11 amino acids, they were the least and most efficient catalysts of NNN 5′-hydroxylation, respectively. The catalytic efficiencies (kcat/Km) for (R)-NNN differed by 170-fold whereas there was a 46-fold difference for (S)-NNN. In general, P450 2As catalyzed (R)- and (S)-NNN 5′-hydroxylation with significantly lower Km and higher kcat/Km values than NPIP or NPYR α-hydroxylation (p < 0.05). Furthermore, P450 2As were better catalysts of NPIP α-hydroxylation than NPYR. P450 2A4, 2A5, 2A6, and 2A13 exhibited significantly lower Km and higher kcat/Km values for NPIP than NPYR α-hydroxylation (p < 0.05), similar to previous reports with P450 2A3. Taken together, these data indicate that critical P450 2A residues determine the catalytic activities of NNN, NPIP, and NPYR α-hydroxylation.

Original languageEnglish (US)
Pages (from-to)61-69
Number of pages9
JournalChemical research in toxicology
Volume18
Issue number1
DOIs
StatePublished - Jan 2005

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