Abstract

We describe the kinetics of pentoxifylline formation from lisofylline in human liver microsomes using selective inhibitors of cytochrome P450 isozymes, correlation studies with specific isozyme activities, and cDNA-expressed human CYP1A2 and 2E1. A biphasic model fitted the data best for the formation of pentoxifylline,K_m1 = 0.282 ± 0.135 μM, V_max1 = 0.003 ± 0.001 nmol/min/mg protein, K_m2= 158 ± 42.6 μM andV_max2 =0.928 ± 0.308 nmol/min/mg (N = 4). Pentoxifylline formation by the lowK_m isoform (200 μM lisofylline) required NADPH, was not inhibited by any isozyme-specific P450 inhibitor, and was inhibited only 10% and 20%, respectively, by aminobenzotriazole and N-octamylamine. We concluded that the low K_m enzyme was not a cytochrome P450. At 5 μM of lisofylline the CYP1A2 inhibitor, furafylline, inhibited pentoxifylline formation by 58.8%, and the nonspecific CYP2E1 inhibitor, diethyldithiocarbamate, inhibited pentoxifylline formation by 21.7%. When preincubated with furafylline plus diethyldithiocarbamate, inhibition of pentoxifylline formation was increased 71.4%. Microsomal CYP1A2 activity correlated with pentoxifylline formation (r² = 0.870,p < 0.001). However, CYP2E1 activity did not correlate with pentoxifylline formation (r² = 0.143, p = 0.181). Baculovirus insect cell expressed human CYP1A2 formed pentoxifylline at 0.987 nmol/min/nmol cytochrome P450 at 5 μM lisofylline. cDNA expressed CYP2E1 did not catalyze formation of pentoxifylline. Diethyldithiocarbamate inhibited pentoxifylline formation by 85.7% in cDNA expressed CYP1A2. We conclude that CYP1A2 is the high affinity enzyme catalyzing pentoxifylline formation from lisofylline.