are much lower than the catfish UDPGA levels measured with this method. Intestinal

UDPGA concentrations in the catfish reported here are the first, to our knowledge, ever

to have been reported for the intestine of any piscine species. These concentrations were

in the same range as that reported for rats, but higher than humans and lower than guinea

pigs. While some of these differences are species-related, another important contributor

to the discrepancy is the different analytical techniques, some of which are inherently

limited by an indirect measurement of UDPGA. Another possible source of variation may

have been the dietary status of the individual animal, since UDPGA concentrations are

decreased by fasting (Reinke et al., 1981). The only values measured in fish are those

measured by Zhivkov and co-workers (1975), who homogenized liver tissue in perchloric

acid in order to solubilize the nucleotides. This may have led to the lower values

observed in trout and carp liver (Table 6-1) relative to catfish liver, since the rate of

hydrolysis of UDPGA to UDP has been shown to be proportional to hydrogen ion

concentration (Bedford et al., 2003).

 Hepatic UDPGA concentrations were in the range of 329-444 C1M. The UDPGA

Km values obtained for the hepatic glucuronidation were 247 CIM and 697 CIM for 4'-

OHCB-72 and 4'-OHCB-35 respectively (Table 4-1). This means that, in vivo, at the

saturating concentration of substrate used in the assay, and assuming that this UDPGA

concentration range is typical of catfish populations, hepatic glucuronidation proceeds at

a suboptimal rate for both 4'-OHCB-72 and 4'-OHCB-35. Of course, one must remember

that the substrate concentrations used in the assay were not representative of

environmental concentrations, and thus at smaller, more realistic OH-PCB levels, the

UDPGA concentration is probably sufficient to efficiently glucuronidate 4'-OHCB-35.