Kinetic constants of lactase of the small intestine of growing rats fully recovered from protein depletion, in relation to dietary protein and lactose concentration.

The effect of dietary composition on the kinetic constants of intestinal lactase was studied using rats depleted of protein by feeding protein-free diet from the weanling stage to 34 days of age and subsequently allowed to recover on diets containing 11.5 or 17% of protein calories (P%) and different levels of lactose (L%; 1, 15, 27 and 37%). After four days of refeeding, rats were decapitated and lactase activity was determined at different substrate concentrations by the method of Dahlqvist using homogenate of intestinal mucosa. Maximum velocity (Vmax) and Michaelis constant (Km) were calculated according to Eisenthal and Cornish-Bowden. At both levels of P%, Vmax tended to increase with the dietary lactose concentration. With diets containing 37% lactose, at P% 11 Vmax was about 60% of that at P% 17.0 Km tended to increase with L% in groups given the 17.0% protein calorie diets, but no difference was observed between groups fed at the lower level of protein. These results can be explained on the basis of interactions between dietary protein at different concentrations and inducer substrate which results in changes in isoenzyme patterns.