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Irradiation-induced constrictive pericarditis in intestinal lymphangiectasis.

The demonstration by direct measurement that lysine is very poorly absorbed by the small intestine of a cystinuric patient confirms previous work which has indicated that in cystinuria the active transport system for dibasic amino-acids is defective (Milne et al., 1961). In cystinuria there is no clinical evidence of malnutrition despite this defect of intestinal transport, and despite the considerable renal loss of dibasic amino-acids and cystine. An alternative mechanism of absorption, perhaps by diffusion, would therefore appear to be likely. Plasma amino-acid levels after oral tolerance tests do indicate that some absorption of free lysine can occur (Hellier et al., unpublished observations), but this method allows no assessment of its extent. The direct measurements reported in the present paper show that lysine absorption is in fact considerably greater when taken as a dipeptide, and it is very likely that in the cystinuric subject most dibasic amino-acid is absorbed in this way. The extent to which amino-acids are absorbed as free amino-acids, after complete intraluminal hydrolysis of protein, or as oligopeptides after incomplete hydrolysis, has been a disputed question for many years. Recently, however, evidence has been accumulating that not only are dipeptides absorbed both in animals and in man, but that their absorption may be of nutritional significance (Craft et al., 1968; Matthews et al., 1968, 1969). The inherited diseases in which defects of intestinal transport of specific amino-acids occur provide a unique opportunity for studying the mechanism of dipeptide absorption. Dipeptide absorption has not previously been studied in cystinuria, but Asatoor et al. (1970a, 1970b) carried out oral tolerance tests in two patients with Hartnup disease, a very rare condition in which clinically detectable malnutrition occurs due to defective small-intestinal transport of neutral aminoacids. They were able to show convincingly that while there was little or no absorption of neutral amino-acids when the free amino-acid was administered, various dipeptides containing the same amino-acids were readily absorbed. Our findings support their conclusion that in normal subjects amino-acids are absorbed by a dual mechanism either as free amino-acids or as dipcptides. Free dibasic amino-acids are absorbed very slowly from the human jejunum, and as in animals very low transport constants (Kt and Vmixax) are found (Hellier et al., unpublished observations). In the one normal subject studied lysine was better absorbed as the dipeptide than as free-amino acid, and it is quite possible that absorption of dibasic amino-acids as dipeptides might be of nutritional significance in normal subjects. In animals a very limited number of dipeptides have now been studied, but it would appear that the relative rate of absorption as free amino-acid or as dipeptide differs considerably for different dipeptides (Asatoor et al., 1970b). The same is probably true in man, as Asatoor et al. (1970b) showed apparently better absorption of phenylalanine as free amino-acid than as dipeptide, a finding in contrast with our observations on lysine, and Matthews et al. (1968) showed diglycine to be absorbed more rapidly than glycine. The site of hydrolysis of oligopeptides into the constituent amino-acids is unknown, though as only free amino-acids are found in portal vein blood it is reasonable to assume that the peptidases lie somewhere in the mucosal cell. It is of interest that during the dipeptide perfusion in the normal subject only very small quantities of glycine and lysine were detected in the recovered perfusate. In the cystinuric subject, though minimal quantities of free glycine were detected, lysine was found in higher concentrations. The source of this lysine is uncertain. It may have resulted from intraluminal hydrolysis of the dipeptide, free glycine being readily absorbed while lysine accumulated within the lumen. Alternatively, free lysine may have diffused back into the intestinal lumen after dipeptide absorption and intracellular hydrolysis. We are grateful to Professor E. F. Scowen for allowing us to study patients under his care, to Mr. C. Thirumalai for technical help, and to our subjects for their co-operation. Requests for reprints should be addressed to Dr. M. D. Hellier.