ABSTRACT
The rate of retinoic acid catabolism was substrate-dependent with free retinoic acid or with holo-CRABP (Figure 18). The Michaelis constants were 49, 53, and 44 nM for total metabolites, peak I, and peak II, respectively, from eH]retinoic acid, with Vmax values of 6.7, and 3.3 pmol/min/mg of microsomal protein, respectively. With holo-CRABP, the Michaelis constants were 0.8, and 2.4 nM for total metabolites, peak I, and respectively, with Vmax values of 2.6, 1.1, and 1.5 pmol/min/mg of protein, respectively. For generating the latter data a 3 to 1 molar ratio was used of CRABP to eH]retinoic acid. Under these conditions, the concentration of unbound retinoic acid present would vary from 2.2 nM, with a total retinoic acid concentration of 5 nM to 4.6 nM, with a total
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