Sucrase-alpha-dextrinase in the rat. Postinsertional conversion to inactive molecular species by a carbohydrate-free...

Absence of dietary carbohydrate decreases both activities of intestinal brush border sucrase-alpha-dextrinase. We examined the molecular mechanism causing this decrease. Adult rats were fed chow (70% CHO) or matched carbohydrate-free (CHO-free) diet for 7 d. Sucrase activity decreased by 50% in whole homogenates and brush borders. Enzyme kinetics revealed no change in sucrose affinity (CHO-free Km = 18 mM, chow Km = 21 mM), but fewer active sites (CHO-free Vmax = 2,720, chow Vmax = 5,000 mumol/min per g protein). Intraintestinal pulse-labeling of [35S]methionine in vivo revealed no differences in incorporation into sucrase. Immunoreactive sucrase protein, assayed by ELISA and rocket immunoelectrophoresis, increased twofold per milliunit of sucrase enzymatic activity in CHO-free jejunum. Total immunosucrase (St), the sum of active and inactive enzyme (St = Sa+Si), was unchanged with carbohydrate withdrawal, but > 50% of the sucrase protein became inactive. SDS-PAGE of sucrase immunoprecipitates revealed alteration of alpha, beta, and gamma subunits in CHO-free animals: (a) alpha and beta subunits migrated farther (mass change--2 kD); and (b) the alpha subunit became diffuse or was a doublet and was less abundant than the beta subunit. Rather than representing loss of sucrase protein, the decline in sucrase activity is achieved with structural subunit changes, probably involving postinsertional processing.