M. L. G. Gardner

INTESTINAL ASSIMILATION OF INTACT PEPTIDES AND PROTEINS FROM THE DIET-A NEGLECTED FIELD?

CONTENTS

Cysteine: A potential source of error in amino acid analysis of mercaptoethane sulfonic or hydrochloric acid hydrolysates of proteins and peptides

Hydrolysis of proteins and peptides with mercaptoethane sulfonic acid is liable to produce overestimation of the proline content owing to the production of ninhydrin-positive material (probably cysteine) which coelutes with proline on many ion-exchange analytical systems. A similar error occurs with HCl hydrolysis (especially in the presence of mercaptoethanol or thioglycollic acid) if care is not taken to oxidize cysteine during reconstitution of the hydrolysate before amino acid analysis.

Entry of Peptides of Dietary Origin into the Circulation

Current theories of protein digestion and absorption state that free amino acids are the only significant form in which amino-N crosses the intestine and enters the systemic circulation. Nevertheless, there is now abundant evidence that intact peptides escaping digestion (or undergoing only partial digestion) and even whole protein molecules can enter the circulation in health and disease. Since peptides are now believed to play a major role in neural and endocrine regulation and in view of the fact that intestinal digestive activity can be reduced and intestinal ‘permeability’ increased in a variety of pathological conditions, the possibility that absorbed intact peptides can exert biological activities in peripheral tissues and can elicit symptoms in susceptible individuals now requires critical evaluation. These considerations may provide a physiological rationale for changing the source(s) of dietary protein in cases where food allergies/dietary sensitivities are suspected.

Can Elemental Diets Reduce the Intestinal Toxicity of 5-Fluorouracil?

In order to test the hypothesis that elemental diets protect the intestinal mucosa against 5-fluorouracil toxicity, we have estimated water absorption, cytoplasmic peptide hydrolase activities, and mucosal DNA contents in isolated intestines from rats fed on one of three elemental diets or a standard diet and injected with 5-fluorouracil. Water absorption rates were significantly increased when rats, not injected with 5-fluorouracil, were fed on Flexical or Vivonex-HN. However, water absorption was severely impaired 3 days after administration of 5-fluorouracil and none of the elemental diets alleviated this impairment of absorption. Cytoplasmic peptide hydrolase activities and mucosal DNA contents were also severely decreased after 5-fluorouracil injection, the changes observed being independent of the diet. Also, none of the elemental diets reduced the body weight losses observed after 5-fluorouracil administration. Although the effects of the sodium salt of 5-fluorouracil on body weight and food intakes were much less severe than those of the Tris salt the two salts had identical effects on intestinal absorption even when an elemental diet was used. Although there was a smaller incidence of diarrhea after 5-fluorouracil administration when the rats had been fed on one of the elemental diets, diet B, the results do not support the suggestion that elemental diets might reduce the intestinal toxicity of 5-fluorouracil.

The differential effects of two salts of 5-fluorouracil and of dietary restriction on absorption by rat small intestine

Abstract Water absorption, measured in vitro in the perfused rat small intestine, following injection of the Tris or sodium salt of 5-fluorouracil (5FU) has been compared with that following a food restriction regime which mimicked the food intake pattern of rats after injection of 5FU. Water absorption rates were decreased both after administration of 5FU and after food restriction but the decrease following 5FU injection was much greater. Also, the Tris and sodium salts of 5FU had different effects on the food intake pattern after injection but had very similar effects on intestinal absorption of water. Hence the effects of 5FU on absorption cannot wholly be attributed to a reduced food intake. Four reference systems have been used to express water absorption rates: whole small intestine, unit length of intestine, intestinal dry weight and mucosal DNA content. The importance of the choice of reference system is discussed. Due to the heterogeneity of the mucosal cell population the use of DNA and dry weight as reference systems could not give an unequivocal indication of whether the changes in absorption rates resulted from a change in the number of absorbing cells or from a change in the absorptive capacity of each absorbing cell. Severe systemic toxicity, including death, was observed after injection of the Tris salt of 5FU, but not after injection of the sodium salt.

Multiple actions of phenylethylbiguanide on respiration by rat liver mitochondria.

Abstract The effect of phenformin over a wide range of concentrations on oxidation of several subtrates by rat liver mitochondria was studied polarographically. Biphasic responses with both stimulatory and inhibitory phases were observed: their order of appearance depended on the nature of the substrate. Dose-response data for these phenomena are reported. At low phenformin concentrations ( −4 M) only stimulation occurred, while at high concentrations (> 10 −2 M) only inhibition was observed. Insensitivity to dinitrophenol and ADP suggested that the stimulation was due to progressive uncoupling. Very high concentrations of phenformin did not inhibit the oxidation of ascorbate/TMPD. Possible sites of action of phenformin are discussed. Although it is not possible to define these with certainty it appears that several sites must be involved.

A comparison of internal and external standardization in amino acid analysis

To assess quantitatively the merits of internal standardization, an amino acid mixture of known composition has been analyzed by conventional automated amino acid analysis before and after being subjected to total acid hydrolysis. Both the precision (reproducibility) and the accuracy (lack of bias) were generally, but not invariably, greatly enhanced by the use of internal standardization with norleucine as opposed to external standardization. Incorporation of internal standards into samples is strongly recommended.