Nutrient absorption.

Some interesting advances in mechanisms and regulation of nutrient absorption were reported last year. Further evidence was obtained that the rate-limiting step in triacylglycerol absorption, especially with large doses of lipid, is transport of prechylomicrons from the endoplasmic reticulum to the Golgi apparatus. Targeted disruption of the adenosine triphosphate-binding cassette transporter in mice produced changes similar to human Tangier disease and suggested that this mouse may be a model for studying intestinal high-density lipoprotein assembly and secretion. A new mechanism for carbohydrate malabsorption was discovered: in sucrase-isomaltase deficiency, the enzyme fails to anchor in the brush border membrane and so is secreted into the lumen, where it is ineffective. Glycosylating insulin at B1 phenylalanine permitted it to bind to the brush border membrane and greatly enhanced its hypoglycemic activity when given orally. CaCo-2 cells and normal human enterocytes were shown to have two variants of the human sodium-dependent vitamin C transporter, hSVCT1; one is active and the other is an inactive splice variant. In rats, the divalent metal ion transporter, DMT1, appeared to be important for regulation of both absorption of iron and its movement into the liver.