The effect of 20(OH)D3 concentration on production of products A–E by mouse liver microsomes. 20(OH)D3 at concentrations varying from 6.25 to 200 μM, dissolved in cyclodextrin at a final concentration of 0.45%, were incubated with the mouse liver microsomal fraction. Incubations were carried out for 30 min at 37 C, the reaction stopped with dichloromethane, 22(OH)D3 (1.0 nmol) added as internal standard, then products extracted and analyzed using HPLC Program A (see Experimental Procedures). Data were fitted to the Michaelis-Menten equation.