A comparison of beta-carotene-splitting activity isolated from intestinal mucosa of pasture-grazed sheep, goats and cattle.

The in vitro enzymatic conversion of beta-carotene to retinal by partially purified preparations from the intestinal mucosa of sheep, goats and cattle was demonstrated. At pH 7.8 and 37 degrees C, this enzyme from sheep, goats and cattle displayed Michaelis-Menten type kinetics with a Vmax (nmol substrate split/mg protein/h) of 0.21, 0.27, and 0.04 and an apparent Km of 4.66 x 10(-6) M, 13.4 x 10(-6) M and 1.27 x 10(-6) M respectively. Maximal reaction was obtained by addition of an appropriate combination of detergent and bile salt, sodium dodecyl sulphate and egg lecithin. The cattle enzyme was inhibited by sodium glycocholate. The activity of sheep enzyme was higher (P < 0.05) than that of goats and cattle. The measured activity of the preparations from sheep, goats and cattle grazed on the same pasture reflected their contents of the reaction product, retinol, in the liver.