suggesting these two pathways of Phase II metabolism compete at similar 3-OH-B[a]P

concentrations. However, the apparent maximal rate of sulfonation was about 7.5 times

lower than the rate of glucuronidation.

 It was previously reported that the maximum rate of glucuronidation of 3-OH-

B[a]P by polar bear liver was 1.26 nmol/min/mg, or around half the V;;;a value obtained

in this study (Sacco and James 2004). However, the preceding study utilized 0.2 mM

UJDPGA, which, as seen from Table 3-2a, is equivalent to the K,; (for UDPGA) of the

low-affinity enzyme, and thus does not represent saturating concentrations of the co-

substrate. The affinity of the enzyme for 3-OH-B[a]P did not change significantly with a

20-fold increase in UDPGA concentrations, suggesting that substrate binding is

independent of the binding of co-substrate. The binding of UDPGA was biphasic,

indicating that multiple hepatic UGTs may be responsible for the biotransformation.

Biphasic UDPGA kinetics have also been demonstrated in human liver and kidney for 1-

naphthol, morphine, and 4-methylumbelliferone (Miners et al., 1988a,b; Tsoutsikos et al.,

2004). While V;;;a was similar for both components, there was a fivefold decrease in

enzyme affinity for UDPGA as the co-substrate concentration was increased. The

involvement of at least two enzymes can be physiologically advantageous since it enables

the maintenance of a high turnover rate even as UDPGA is consumed. Although

physiological UDPGA concentrations in polar bear liver are unknown, mammalian

hepatic UDPGA has been determined to be around 200-400 CLM (Zhivkov et al., 1975,

Cappiello et al., 1991), implying that the observed nonlinear kinetics in the polar bear

may operate in vivo.