Question

The uptake of l-ascorbate (vitamin C) and its oxidized form, dehydro-l-ascorbic acid

(DHAA), was evaluated in brush border membrane vesicles isolated from adult human

small intestine. Ascorbate uptake was Na+-dependent and potential-sensitive (Km, 200

μmol/L), whereas DHAA transport occurred through Na+-independent facilitated

diffusion (Km, 800 μmol/L).

If the Vmax of vitamin C import through channels is 506µmol/min/cm², what import rate would you expect if the lumen contained 137 μmol/L of the version of vitamin C that moves through them?

Answer 1

Ans:  This question is solved by applying Michaelis Menten equation given as under

V0 = Vmax x [S]/ Km + [S]------------------Michaelis Menten equation

Given

Vmax = 506µmol/min/cm2

[S] = 137 μmol/L

Km  =  200μmol/L----ascorbate (vitamin C)

Km  =  800μmol/L----dehydro-l-ascorbic acid (DHAA)

Sustitute these values in Michaelis Menten equation, we get

V0 = Vmax x [S]/ Km + [S]

V0 = 506µmol/min/cm2 x 137 μmol/L/ 200μmol/L + 137 μmol

V0 = 205.7µmol/min/cm2 (Import rate of the l-ascorbate (vitamin C) at  Km  =  200μmol/L)

V0 = Vmax x [S]/ Km + [S]

V0 = 506µmol/min/cm2 x 137 μmol/L/ 800μmol/L + 137 μmol

V0 = 73.98 µmol/min/cm2 (Import rate of the dehydro-l-ascorbic acid (DHAA) at Km  =  800μmol/L)