Food proteins and gut mucosal barrier. II. Differential interaction of cow's milk proteins with the mucous coat and the surface membrane of adult and immature rat jejunum.

Two in vitro intestinal models were used to investigate postnatal maturational changes of the gut barrier functions. Microvillus membrane (MVM) preparations were studied for surface binding, and everted gut sacs were studied for mucous coat binding, breakdown and uptake of radioiodinated bovine serum albumin (BSA), and beta-lactoglobulin (beta-LG). Surface binding of these proteins to MVM was weak and nonspecific. There was more binding of both proteins to immature MVM (BSA: newborns, 2.74 +/- 0.52%, adults, 1.08 +/- 0.17%, p less than 0.001; beta-LG: newborns, 6.30 +/- 0.54%; adults, 2.05 +/- 0.07, p less than 0.001). In contrast to MVM binding characteristics, mucous coat binding of the cow's milk proteins to immature gut sacs was significantly less (BSA: preweanlings, 0.94 +/- 0.30 micrograms 125-I-protein Eq/mg mucosal protein; adults, 3.06 +/- 0.74, p less than 0.001; beta-LG: preweanlings, 5.61 +/- 1.48; adults, 9.83 +/- 1.33, p less than 0.001). Protein binding and uptake were correlated in the immature animals (r = 0.76, p less than 0.001 for BSA and r = 0.85, p less than 0.001 for beta-LG). More beta-LG was bound and taken up than BSA in the preweanlings (p less than 0.001). Trichloroacetic acid precipitation studies showed that, even in the immature rats, beta-LG was much more readily broken down by mucosa-associated enzymes than BSA. Immature animals showed less protein breakdown than adult controls. Decreased protein breakdown and mucous coat binding as well as increased MVM binding may account for the increased uptake of intact food antigens in the newborn.(ABSTRACT TRUNCATED AT 250 WORDS)