The intestinal absorption of amino‐acid enantiomorphs

Abstract

Gibson & Wiseman (1951) showed that when amino-acids were injected into the lumen of the intestine of the rat, the L-enantiomorph disappeared at a faster rate than the D-enantiomorph, and they postulated a stereochemically specific mechanism for the absorption of L-amino-acids. In support of this, Clarke, Gibson, Smyth & Wiseman (1951) showed that L-amino-acids also disappeared faster than D-amino-acids from the Thiry-Vella loop in the dog, and Wiseman (1953) showed with an in vitro technique that L-amino-acids could be transferred by the intestine against a concentration gradient, whereas D-amino-acids could not. While these experiments furnish strong evidence for the existence of a stereochemicaily specific mechanism for absorption of amino-acids from the intestine, they fail to supply any information about the concentration of the D-and L-amino-acids in the blood stream during absorption, and the present work was undertaken to investigate this aspect of the problem. Preliminary accounts of the results have been given by Matthews & Smyth (1952, 1953. METHODSThe experiments were done on cats anaesthetized with pentobarbitone sodium. The carotid artery was cannulated by the procedure described by Crampton & Smyth (1953), which enables blood to be collected from or introduced into the carotid artery with simultaneous measurement of the blood pressure. A loop of the intestine was selected near to the caecum and spread out so that the arterial and venous supply could be inspected. A part of the intestine drained by a suitable vein was chosen, and this section of intestine was cannulated, washed out with warm saline (0-9 % NaCI) and tied off at each end. The vein draining the loop was now prepared for cannulation without any interference with the blood flow. There was thus an isolated section of intestine about 30 cm long with intact blood supply, and so arranged that on cannulation of the vein draining the loop, the blood from this section of the intestine could be collected. The loop of intestine was arranged on a heated plate and protected with a Perspex cover, so that it could be kept at a temperature of 370 C, moistened with saline and inspected during the experiment.The animal was then heparinized and the blood pressure measured in the carotid artery. Blood (30-50 ml.) was withdrawn from the carotid artery till the blood pressure had fallen to about 50 mm of mercury. The object of this was to reduce the rate of blood flow. Although this must

Competition between enantiomorphs of amino acids during intestinal absorption

“…The D-and L-enantiomorphs of methionine and histidine were obtained commercially. As will be shown, Gibson & Wiseman (1951) and Matthews & Smyth (1954) in showing that the L-enantiomorphs are absorbed more rapidly than the corresponding D-forms. The present results, however, give figures for absorption rates when only the L-or D-enantiomorph was present in the intestine, whereas in the other experiments referred to the amino acid was present as the racemic mixture.…”

“…Gibson, Newey, Smyth & Whaler (1954) showed that the capacity for synthesis of some L-amino acids from the corresponding D-enantiomorphs was not present in the intestine to any appreciable extent, although in fact histidine was not one of the amino acids which they tested. Certainly all transfer of D-amino acids does not happen by conversion to the L-amino acid, since Matthews & Smyth (1954) found that D-amino acid appeared in the portal blood when present in the lumen of the intestine.…”

The intestinal absorption of glucose

“…The principle of the method used was that ofMatthews & Smyth (1954) in which the blood draining a loop ofintestine is collected, so that none of the products of absorption entering the blood stream are returned to the animal. In this method, the blood pressure was maintained at a low level in order to reduce the rate of intestinal blood flow.…”

In Vitro Investigation of D- and Lenantiomer Synergistic Efects of Some Amino Acids

“…GIBSON and WISEMAN (1951) reported that L-forms of amino acids disappeared from intestine of rats more rapidly than their corresponding D-enantiomers and therefore they regarded existence of a stereochemically specific mechanism for active absorption of L-amino acids. Later results (VISEMAN, 1953;AGAR et al, 1953;MATTHEWS and SMYTH, 1954) showed that transport of L-amino acids involves an active process and it was assumed that this did not apply to D-amino acids. On the other hand, JERVIS and SMYTH (1959) showed that competition between some Dhistidine and L-Met was possible using in vivo as well as in vitro techniques (WISEMAN, 1955).…”