Renal microsomal N-nitrosodimethylamine demethylase determined by a sensitive radiometric method

Area of Science:

• Biochemistry
• Pharmacology
• Toxicology

Background:

• N-nitrosodimethylamine (NDMA) is a potent carcinogen.
• Understanding its metabolic pathways, particularly N-demethylation, is crucial for risk assessment.
• Renal and hepatic microsomal enzymes play a key role in xenobiotic metabolism.

Purpose of the Study:

• To quantify and compare renal microsomal N-nitrosodimethylamine (NDMA) N-demethylation activity across different species.
• To investigate the cofactor dependency and kinetic properties of NDMA demethylation in liver and kidney.
• To identify potential species-specific differences in NDMA metabolism.

Main Methods:

• A sensitive radiometric assay using [14C]NDMA as a substrate was employed.
• [14C]Formaldehyde, a product of NDMA demethylation, was trapped using 2,4-dinitrophenylhydrazine (DNPH).
• Enzyme kinetics, cofactor preferences, and inhibitor effects were analyzed.

Main Results:

• Significant species-dependent variations in NDMA N-demethylation activity were observed.
• Mice exhibited the highest activity, while hamsters showed the lowest; rats displayed undetectable activity.
• The DDY mouse strain demonstrated the highest demethylation capacity among studied mouse strains.
• Nicotinamide adenine dinucleotide (NAD+) supported demethylation more effectively in kidney (32%) than in liver (16%) compared to nicotinamide adenine dinucleotide phosphate (NADP+).
• Kinetic properties and inhibitor responses were largely similar between kidney and liver microsomes.

Conclusions:

• Renal microsomal NDMA N-demethylation is highly species-specific, with notable differences between mice, hamsters, and rats.
• The DDY mouse strain is a suitable model for studying NDMA metabolism due to its high activity.
• While cofactor preference differs, the overall enzymatic characteristics of NDMA demethylation in kidney and liver share similarities.