TABLE 5

Noncompetitive isotope effects on ratios of formation rates (effects significantly different from 1.0 (P < 0.01)

Commercial CYP1A2 microsomes supplemented with reductase and b5 were the enzyme source and incubations were performed as described under Materials and Methods. Turnover numbers are presented as nmol min–1 nmol P450-1. Estimations of isotope effects were calculated from the mean ± S.D. for each d0 metabolite (n = 3) divided by the mean ± S.D. for each d3 metabolite (n = 3).

Site of Metabolism
d0-Caffeine
d3-Caffeine
d0-Caffeine/d3-Caffeine
DV ([caffeine] = 5 mM)
    N-3 (PX) 1.01 ± 0.08 0.093 ± 0.007 10.8 ± 1.14a
    N-1 (TB) 0.133 ± 0.006 0.186 ± 0.001 0.72 ± 0.03a
    N-7 (TP) 0.081 ± 0.004 0.083 ± 0.006 0.98 ± 0.01b
    C-8 (TMU) N.D. N.D.
    Total metabolism 1.22 ± 0.08 0.362 ± 0.009 3.37 ± 0.23a
    Ratio (PX/TB/TP) 12.5: 1.6: 1.0 1.1: 2.2: 1.0
DV/K ([caffeine] = 100 μM)
    N-3 (PX) 0.107 ± 0.002 0.014 ± 0.002 7.69 ± 0.97a
    N-1 (TB) 0.016 ± 0.001 0.024 ± 0.001 0.67 ± 0.07a
    N-7 (TP) 0.008 ± 0.000 0.007 ± 0.001 1.16 ± 0.08c
    C-8 (TMU) N.D. N.D.
    Total metabolism 0.131 ± 0.002 0.045 ± 0.002 2.93 ± 0.06b
    Ratio (PX/TB/TP)
13.4: 2.0: 1.0
2.0: 3.4: 1.0

  • N.D., not detected.

  • ↵ a Ratios are significantly different from 1.0 (P < 0.005).

  • ↵ b 0.1 < P < 0.25.

  • ↵ c 0.025 < P < 0.05.