Discussion

 Two major strategies have been used to study UDPGA concentrations in tissues.

Indirect determination of UDPGA concentrations is based on the normally linear

relationship between glucuronide formation and UDPGA concentration. The

determination of glucuronide formation, whether via radiochemical detection (Schiller et

al., 1982; Watkins and Klaasen, 1982; Hjelle et al., 1985; Cappiello et al., 1991),

fluorometry (Singh et al., 1986) or reverse-phase HPLC (Yamamura et al., 2000), can

then be used to down-extrapolate the UDPGA level via a standard curve. Since this

method assumes that the linear relationship holds at low UDPGA physiological

concentrations, determination of this co-substrate in tissues with lower levels (such as

intestine) may be more subj ect to inaccuracies.

 Reverse phase HPLC has been used to directly determine UDPGA concentrations

in liver cell extracts (Aw and Jones, 1978; Dills et al., 1987; Alary et al., 1992) and whole

tissue (Adachi et al., 1991; Suto et al., 2002). Imamura and co-workers (2003) used a

reverse-phase system in order to determine both UDPGA and PAPS in cultured rat

hepatocytes. While direct determination of UDPGA by HPLC is most desirable, the use

of a C18 reverse-phase column was unsuccessful, due to the interference of other

substances co-eluting with the UDPGA peak, as well as a drifting baseline. The use of an

anion-exchange HPLC column dramatically improved resolution, sensitivity and

reproducibility. Sub-micromolar concentrations of UDPGA standard, dissolved in

ammonium phosphate buffer, could be detected.

 Catfish liver UDPGA concentrations were similar to those previously reported for

mammals such as humans, rats, and guinea pigs (Table 6-1). The results reported by

Zhivkov and co-workers (1975) for other mammalian species, birds, amphibians and fish