A novel intracellular pathway for rat intestinal digestive enzymes (alkaline phosphatase and sucrase) via a lamellar particle

To identify the mechanism of intestinal alkaline phosphatase (IAP) release into serum that is known to be associated with fat feeding, both luminal and serum IAP as well as sucrase-isomaltase levels were monitored by rocket electroimmunoassays and by immunogold labeling electron microscopy. Luminal and serum IAP, and to a much lesser extent sucrase-isomaltase, peaked at 7 hours after fat feeding. Analysis of the luminal IAP by isoelectric focusing showed that the enzyme had a slightly different pI than brush border IAP, but was still partially membrane bound, whereas serum IAP was no longer membrane bound. In parallel, by morphology, IAP and sucrase-isomaltase localized to intra- and extracellular lamellar membranous particles, most conspicuous at 7 hours after fat feeding, while very scarce in nonfat-fed animals. The membranous particles bearing the enzymes were commonly associated with fat droplets. These data are consistent with the existence of a novel pathway, possibly secretory, for IAP and sucrase-isomaltase via lamellar bodies, leading to appearance of the enzymes in lumen and serum, with subsequent release from the membrane extracellularly. They also offer an explanation for the known association of the increased secreted IAP after fat feeding.