The biosynthetic basis of adult lactase deficiency.

Abstract

The intestinal brush-border enzyme lactase splits lactose into its component monosaccharides, glucose and galactose. Relative deficiency of the enzyme during adulthood is a common condition worldwide and is frequently associated with symptoms of lactose intolerance. We studied the synthesis and processing of lactase in normal and adult hypolactasic subjects using human intestinal explants in organ culture. Metabolic labeling experiments in our control subjects with I35Slmethio-nine followed by immunoprecipitation, sodium dodecyl sulfate-polyacrylamide-gel electrophoresis, and fluorography demonstrated that newly synthesized lactase is initially recognized as a precursor molecule with a relative molecular weight (Mr) of 205,000. Over the course of several hours most of the labeled lactase was converted to a mature form of 150,000 Mr. Transiently appearing forms of 215,000 and 190,000 M, were identified and were felt to represent intermediary species generated during intracellular processing.We identified two distinct alterations in lactase biosynthesis accounting for adult hypolactasia. Studies in three deficient subjects demonstrated markedly reduced synthesis of the precursor protein though posttranslational processing appeared identical to normal. Multiple studies in a fourth deficient subject demonstrated synthesis of ample amounts of precursor lactase but reduced conversion to the mature active form of the enzyme. (J. Clin. Invest. 1990Invest. . 86:1338Invest. -1342

Regulation of intestinal lactase in adult hypolactasia.

“…Fig. 2 demonstrates a linear relationship between synthesis and mucosal lactase-specific activity reinforcing our prior conclusion that, in general, the level of the enzyme in the intestine is dictated by synthetic rate (4). Accordingly, the reduced incorporation of radioactivity into LPH precursors characterizes subjects 1 and 4 as type I deficients.…”

“…Metabolic labelling studies were performed as previously reported by us (4,7,8). Briefly, duodenal biopsies were equilibrated for 1 h in tissue culture medium, and then continuously exposed to 400 ,Ci [35S]methionine for periods of 1, 5, and 20 h. The radiolabelled explants were homogenized in PBS with 1% Triton X-100 pH 7.5.…”

“…Triton X-100-solubilized homogenates were centrifuged at 27,000 g for 1 h. The supernatants were precleared with Pansorbin (Calbiochem-Behring Corp., La Jolla, CA), and their volumes adjusted to contain equal amounts of radiolabelled total protein. Immunoprecipitation was carried out by adding an excess of monoclonal antibodies (H3 1C4C3), directed against human LPH, to the precleared, volume-adjusted samples (4). The adequacy ofprecipitation was verified by lactase assays on samples both before and after immunoprecipitation.…”

“…Those individuals who exhibit physiologic decline in the enzyme activity with maturation are described as demonstrating adult lactase deficiency. The biosynthesis of LPH in the setting of adult lactase deficiency has recently been studied and two distinct phenotypes reported (4,5). Phenotype I is characterized by reduced synthesis ofprecursor LPH while the mechanism of decreased lactase activity in phenotype II is associated with ample precursor synthesis but reduced conversion of the protein to its mature molecular form.…”

Molecular basis of lactase levels in adult humans.

“…Witte et al. (17), identified markedly reduced synthesis in three deficient patients with low lactase enzyme levels, and decreased conversion of precursor to mature lactase in a fourth deficient patient. Together, these studies suggest that (a) the major determinant of lactose "intolerance" is reduced synthesis of lactase enzyme, and (b) within racial groups which commonly display high lactase activity, there are individuals with specific defects in synthesis and processing, such as have been identified for sucrase-isomaltase deficiency (18).…”