摘要:

At least six urinary metabolite ratios of caffeine have been proposed as probes for in vivo CYP1A2 activity and three for in vivo NAT2 activity. Claims for the frequency distribution of the activity of CYP1A2 based on these empirical ratios have varied from log-normal to trimodal. We have examined the validity of these nine ratios by developing computer simulations using values reported in the literature for the kinetic parameters of caffeine and its metabolites. The results show that the sensitivity of the ratios to confounding variables is, in some cases, greater than their sensitivity to the activity of the enzyme that they are intended to mark. The six CYP1A2 ratios did not exhibit the same pattern of dependency on confounding variables which, in turn, resulted in different shapes of population distributions for each ratio as enzyme activity was varied systematically. Although the dependency of the three NAT2 ratios on confounding variables was less marked, they also showed different patterns of dependency. The outcomes of the simulations were consistent with much of the experimental data on caffeine metabolite ratios. To support the findings from the simulations, simplified equations for each metabolite ratio were derived which emphasize the dominant determinants. With some of the CYP1A2 ratios urine flow was significant to the point where its variance and heterogeneity between populations could lead to spurious detection of polymorphism in CYP1A2 function. Also, if the variability of a dominant confounding factor was high and sensitivity of the ratio to intrinsic CYP1A2 activity was low, any polymorphism in the latter would be obscured. When a specific time interval was defined for urine collection, this time was shown to be a critical factor in the ability to discriminate bimodality in some of the ratios, when a marked polymorphism in enzyme activity was assumed. Those ratios which have shown no evidence for bimodality in CYP1A2 function in experimental studies are inherently more discriminant of such heterogeneity compared to those ratios which have been claimed to detect polymorphism of CYP1A2 from experimental data. While recommending a ‘best buy’ from amongst the caffeine urinary metabolite ratios, we favour plasma/saliva indices (caffeine half-life or paraxanthine/caffeine ratio in a spot sample).

ISSN:0960-314X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Genetic polymorphisms in the activation or detoxication of carcinogens, such as those in tobacco smoke, may produce differences in individual susceptibility to lung cancer. The cytochrome P450 CYP2E1 is an enzyme involved in the metabolism of nitrosamines in tobacco smoke. A polymorphism of CYP2E1 detectable by the restriction enzyme Rsa I may be functionally important becaue it is located in a putative binding site for the transcription factor HNF-1 and has been associated with higher levels of CYP2E1 transcription. It is conceivable that this CYP2E1 Rsa I polymorphism might contribute to differences in susceptibility to lung cancer. We conducted a case-control study of patients with incident lung cancer and population controls in Los Angeles County to examine the association between the CYP2E1 Rsa I polymorphism and lung cancer risk among African-Americans and Caucasians. Samples of white blood cell DNA sufficient for determination of the CYP2E1 Rsa I genotype by a polymerase chain reaction-based assay were obtained from 341 cases and 706 controls with data on lifetime smoking history. No subjects were homozygous for the CYP2E1 Rsa I rare c2 allele. The rare c2 allele was not associated with an increased risk of lung cancer (adjusted odds ratio, OR=0.72; 95% confidence interval, CI=0.35-1.46). Among the population controls the percentage of subjects carrying the rare c2 allele was lower (p=0.002) among African-Americans (2%) compared with Caucasians (8%). However, the association between the CYP2E1 Rsa I genotype and lung cancer risk did not differ between ethnic groups. There was no important association between the CYP2E1 Rsa I genotype and lung cancer risk in analyses stratified by cell-type, smoking history, gender, occupational asbestos exposure, and dietary intake of antioxidants vitamin C, vitamin E or betacarotene. Due to the low frequency of the c2 allele in these populations, larger studies would be necessary to rule out a modest association between the CYP2E1 Rsa I polymorphism and lung cancer risk.

ISSN:0960-314X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Caffeine is used to phenotype subjects in vivo for the cytochrome P450 isoforms CYP1A2 and CYP2E1, and for N-acetyltransferase type 2 (NAT2). However, how much of the variation in phenotyping parameters may be attributed to variations in CYP1A2 and CYP2E1 activities has not been determined. Therefore, this study intraindividually compared enzyme activities and/or content in liver samples with pharmacokinetic parameters of caffeine in vivo after administration of a test dose in 25 patients undergoing hepatectomy. Parameters measured in vitro were the high affinity components of caffeine 3-demethyIation and phenacetin O-deethylation, microsomal CYP1A2 and CYP2E1 immunoreactivity, and cytosolic sulfamethazine N-acetylation. Caffeine parameters in vivo included caffeine clearance from plasma and/or saliva, paraxanthine/caffeine ratios in plasma and saliva, plasma theophylline/ caffeine ratio, and several metabolite ratios from spot urine sampled 6h postdose. Correlations between parameters were determined using weighted linear regression analyses. Caffeine clearance and paraxanthine/caffeine ratios correlated most highly to intrinsic clearance of caffeine 3-demethylation and to CYP1A2 immunoreactivity (r=0.58-0.82), whereas urinary CYP1A2 ratios correlated less strongly with CYP1A2 parameters in vitro. Assignment of acetylator phenotype by urinary NAT2 ratios was concordant with sulfamethazineN- acetylation in vitro. In contrast to CYP1A2 paramters in vitro, CYP2E1 immunoreactivity was not related to the theophylline/caffeine plasma ratio. CYP1A2 activity, thus, is the major determinant of caffeine clearance and the paraxanthine/caffeine ratios in vivo, of which the saliva ratio 6 h postdose appears as the most advantageous parameter. The results confirm that phenotyping using caffeine provides valid estimates of CYP1A2 and NAT2 activity.

ISSN:0960-314X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

The hepatic N-acetyltransferase enzyme encoded by theNAT2*gene locus is responsible for the human polymorphic acetylation of numerous arylamine or hydrazine-containing drugs and xenobiotics including AIDS-related therapeutic agents such as isoniazid and sulphonamides. The genetic basis underlying the human acetylation polymorphism has been extensively studied in several populations but native African populations were poorly documented. In the present study, 117 unrelated black Africans, namely Dogons from Mali and Gabonese, were investigated forNAT2*allelic variability and genotype distribution. ThirteenNAT2*alleles were unambiguously identified by combined use of allele-specific reamplifications and restriction endonuclease digestions. Our results confirm the African origin of G191→A substitution in the NAT2 coding region which was previously associated with slow acetylation in African-Americans. The finding of high allelic diversity in the studied populations is consistent with the hypothesis of a single African origin for NAT2-associated polymorphism. Finally, no excess of the slow acetylator phenotype is predicted in these populations, implying no need for fitting NAT2 polymorphism-sensitive therapies to black Africans, compared to Caucasians.

ISSN:0960-314X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Human glutathione S-transferase (GST) M1 and T1 enzymes exhibit genetic polymorphism, with a percentage of normal individuals exhibiting a homozygous deletion of the relevant genes. We established a differential polymerase chain reaction (PCR) technique to simultaneously characterize inactivating mutations responsible for the null alleles ofGSTM1andGSTT1.Primers forGSTM1, GSTT1,and for β-globin (as a positive control) were used to simultaneously amplify all three gene products from leukocyte DNA from 416 normal healthy human volunteers. IdenticalGSTM1andGSTT1genotypes were obtained using nine samples processed either separately or simultaneously forGSTM1andGSTT1.The frequency of the null genotype forGSTM1was higher in whites (114/213 or 53.5% vs 56/203 or 27.6%, p<0.001) and forGSTT1was higher in blacks (49/203 or 24.1% vs 32/213 or 15.0%, p=0.019). The observed frequency of the ‘double null’ genotype for bothGSTM1andGSTT1was not significantly different from that predicted if both polymorphisms were independent (p = 0.102) and did not differ by race (p = 0.120) or sex (p = 0.800). There was a higher frequency of theGSTM1null genotype among females than males (92/202 or 45.5% vs 78/214 or 36.4%, p = 0.049). These results demonstrate that this PCR method is a simple and reliable tool to simultaneously characterize bothGSTM1andGSTT1null genotypes.

ISSN:0960-314X    

出版商:OVID    

年代:1996

数据来源: OVID