CHAPTER 6
 DETERMINATION OF PHYSIOLOGICAL UDPGA CONCENTRATIONS IN
 CHANNEL CATFISH LIVER AND INTESTINE

 UDP-Glucuronic Acid (UDPGA)

 When trying to determine Km (or Sso) and Vmax values for the substrate of interest

in a bisubstrate system such as glucuronidation, one keeps the concentrations of co-

substrate constant. This is done since, in theory, by using different concentrations of co-

substrate one can get an infinite variety of values (apparent vahues) for the kinetic

parameters. Thus, in order to obtain true vahues via the Michaelis-Menten equation, one

has to determine kinetic parameters at saturating concentrations of co-substrate, assuming

that these excess concentrations do not have any other effect on UGTs. Thus, for

example, saturating concentrations of 0.2 mM UDPGA were used in our catfish intestine

glucuronidation experiments (Chapter 4).

 However, the use of excess concentrations of UDPGA present one with two

problems. Are UDPGA concentrations saturating in vivo? How is the enzymatic

efficiency affected, in view of the affinity (shown by Km or Sso) of UGT(s) for UDPGA?

If the physiological UDPGA concentrations are lower than the excess concentration used

in vitro one may expect to observe a difference in kinetic parameters. While some studies

have measured the physiological UDPGA level in tissues (mainly liver), most of this

work has been limited to mammalian species such as humans and rats (Table 6-1).

 The rate of glucuronidation of 3-OH-B[a]P was also found to be dependent on the

endogenous level of UDPGA by Singh and co-workers (1986). This concept of UDPGA