Influence of milk factors on riboflavin uptake by rat intestinal everted sac

The uptake of riboflavin was measured in Krebs' medium and a system consisting of riboflavin-free dialyzed milk (providing milk factors) equalized to Krebs' medium with respect to different salt concentration and pH. The final riboflavin concentration in the medium ranged between 0.1 and 0.5 micromolar. Total riboflavin incorporation was greater in the milk system and the increase ranged between 120 and 134% in comparison with Krebs' medium (100%). The Michaelis-Menten constant (Km) and Vmax for labelled riboflavin uptake were calculated by Lineweaver-Burk plotting of data. Km value decreased from 1.23 micromolar for Krebs' medium to 0.75 micromolar in the presence of milk factor(s). Vmax values were 62 and 60 pmol/100 mg tissue/min for Krebs' and milk systems, respectively. It was concluded that milk factor(s) enhanced intestinal uptake of riboflavin.