Uptake and metabolism of riboflavin-5'-alpha-D-glucoside by rat and isolated liver cells.

The effect of riboflavin alpha-D-glucoside, isolated from rat liver, on the uptake of riboflavin was studied using freshly isolated rat liver cells. The transport characteristics and metabolic fate of the glucoside were also determined using the radioactive compound. The initial (1-min) uptake of 1 mumol/L [3H]riboflavin glucoside (2.90 +/- 0.29 pmol/10(6) cells) was higher than that of 1 mumol/L [3H]riboflavin (1.35 +/- 0.30 pmol/10(6) cells). However, the accumulation of glucoside after 60 min was significantly lower than that of riboflavin. The presence of up to 30 mumol/L glucoside had no significant effect on the initial uptake of [3H]riboflavin (3 mumol/L, 10(9) cells/L). Measurement of the kinetic parameters for glucoside gave an apparent Kt value of 83.4 +/- 12.4 mumol/L and a Vmax of 208.6 +/- 17.9 pmol/(10(6) cells.min). Decreases in the temperature of incubation decreased uptake rate. Replacement of Na+ with other monovalent cations did not affect uptake. The presence of D-glucose (1 mumol/L to 5.5 mmol/L) had no inhibitory effect on uptake of 1 mumol/L [3H]riboflavin glucoside. The results indicate that the transport of riboflavin glucoside may not involve the transport mechanisms for riboflavin or D-glucose. Metabolic studies with isolated hepatocytes showed that the glucoside was hydrolyzed to yield riboflavin upon entry into the cell. The vitaminic efficiency of this compound was tested by feeding it to growing male rats. These experiments indicate that the glucoside and free riboflavin are comparable sources of the vitamin.