The interaction of human and bovine serum proteins with CYP3A in human liver microsomes: inhibition of testosterone 6β-hydroxylation by albumin, α-globulins, α 1-acid glycoprotein and γ-globulins

Abstract

The effects of human and bovine serum proteins on CYP3A activity, using testosterone as the probe substrate, were investigated in human liver microsomes. Serum albumin, α-globulins, and α 1-acid glycoprotein (α 1-AGP) of both species significantly inhibited testosterone 6β-hydroxylation. When the inhibitory effects of serum proteins were compared with serum protein binding data, human α-globulins, with a ratio (relative metabolic activity/unbound fraction) of 0.3, showed higher, and bovine α 1-AGP, with the ratio of 1.4, showed lower inhibitory effects than those expected from protein binding of testosterone. The effects of the other serum proteins were close to those expected from protein binding, according to the free drug hypothesis. The K i values obtained from the Dixon plots were 0.32% (w/v, 48 μM) for human serum albumin (HSA), 0.48% for human α-globulins, and 0.23% (52 μM) for human α 1-AGP. K i values of bovine serum albumin, bovine α-globulins and bovine α 1-AGP were 3–5 times higher than those of the respective human proteins. The results suggest a direct interaction of some of these serum proteins with the active site of the CYP3A isoform. Since the bovine serum proteins showed weaker inhibitory effects than human serum proteins, the wide use of BSA, which is viewed as interchangeable with HSA, needs to be cautioned.