A histochemical study of gut maturation

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The influence of specific chemical factors in the solvent on the absorption of macromolecular substances from the small intestine of the new-born calf

1. The absorption, without change, of [(131)I] and [(125)I]solutes of high molecular weight after duodenal infusion has been measured in anaesthetized calves less than 20 hr old by analysis of lymph collected from the thoracic or intestinal duct.2. Factors present in boiled bovine colostrum whey known to be necessary for the rapid absorption of [(131)I]bovine serum gamma-globulin have now been shown to accelerate the passage of [(131)I]polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K.60) into the lymph in a comparable manner.3. [(131)I]PVP K.30 (mean mol. wt. 40,000) and [(131)I]human serum albumin could be absorbed to some degree in the absence of solvent factors necessary for the absorption of solutes of higher mol. wt. and a large proportion of the solute thus absorbed passed directly into the portal capillaries.4. Lactate and pyruvate and salts of certain lower volatile fatty acids resemble factors in colostrum whey in their facilitation of the absorption of both gamma-globulin and PVP K.60: these active compounds were not however found in colostrum in significant quantities.5. Potassium isobutyrate was the most effective of the compounds tested and at concentrations of 56.7 m-mole/l. generally accelerated absorption to a greater degree than did colostrum whey itself.6. Absorption of both gamma-globulin and PVP K.60 from colostrum whey was characterized by a profuse flow of lymph containing relatively low concentrations of labelled solute. In contrast, when these solutes were fed in solutions containing simple compounds such as potassium isobutyrate they appeared in very high concentrations in the lymph, the flow of which remained relatively scant.7. When [(125)I]PVP was administered in water, little was absorbed. If, however, such an infusion was followed 3 hr later by a duodenal infusion of colostrum, [(125)I]PVP passed into the lymph almost immediately. This response was too rapid for the colostrum to have reached the absorbing cells in the terminal ileum.8. Intravenous infusions of L+lactate have been found to facilitate the absorption of [(125)I]PVP K.60 introduced into the duodenum in water. This indicates that some of the solvent factors which accelerate absorption may reach the terminal ileum via the blood vascular system after they themselves have been absorbed from the upper small intestine.

The digestive function of lysosomes as studies by the turnover of ingested foreign macromolecules

The persistence of small foreign molecules in lysosomes described by Dr Dingle stands quite in contrast with the fast turnover of foreign macromolecules captured by these organelles. Since Dr de Duve chose to talk today about peroxysomes rather than lysosomes, it might perhaps add to the Discussion to present some data related to this important function, i. e. the ability of lysosomes to digest foreign macromolecular material. As part of our study of the penetration of macro­molecules into mammalian cells (Ryser 1968), we have been studying for some time the fate of 131 I albumin ingested by Sarcoma S180 cells in culture (Ryser 1968; Gabathuler & Ryser 1967). Cells are first labelled with albumin under conditions enhancing albumin penetration (Ryser & Hancock 1965). They are then washed and re-incubated in label-free medium. The fate of the ingested albumin can be followed in two ways; first, by measuring as a function of time the decrease in specific activity of the cellular proteins; secondly, by measuring the radioactivity appearing with time in the re-incubation medium. Figure 60 shows a result of the first type. The value given on the logarithmic ordinate at time zero corresponds to the level of albumin uptake obtained during the labelling period. The loss of acid-insoluble label from the cells is represented by the descending straight lines. This loss is biphasic. Although the second phase has a rather flat slope, it cannot be accounted for by an isotope dilution due to cell growth in vitro . Half-lives of ingested albumin can easily be read from such diagrams, and turn out to be of the order of 50 to 90 min. The rate of breakdown can be influenced to some extent by changing conditions of re-incubation as shown by the difference between the two curves of figure 60, due to a different serum concentration in the medium. Simultaneously with the loss of acid-insoluble activity from the cells, there is a pro­gressive appearance of radioactivity in the re-incubation medium. It was of interest to determine the molecular size of the label appearing in the medium by means of gel filtration on Sephadex G 75. Figure 61 shows the elution diagrams of samples taken from the re-incubation medium at three different times. In all three diagrams there is a strong initial peak of activity excluded by the gel and corres­ponding, therefore, either to unchanged or only slightly modified albumin. It is not yet clear whether this peak results from a desorption of surface-bound albumin or to a regurgitation of ingested material or both. In the upper diagram (sample taken at time zero), radioactivity is found in this initial peak only. As time goes on, however, activity of a different form appears in the medium, as indicated by the occurrence of a second peak, which coincides with the elution of the small-sized marker phenol red (PhR). This peak corresponds to free tyrosin or a dipeptide containing tyrosin. It is evident, therefore, that ingested albumin is broken down not only rapidly, but also thoroughly to the level of free amino acids or dipeptides. Similar evidence has been obtained with phagocytic cells (Mego, Bertini & McQueen 1967; Ehrenreich & Cohn 1968; Unanue & Askonas 1968). Electron microscopy of normal and tumour cells indicates that ingested foreign protein such as ferritin, haemoglobin, peroxydase and 125 I-albumin are essentially localized in secondary lysosomes (multivesicular bodies, phagosomes) (Maunsbach 1966; Neustein & Maunsbach 1966; Friend & Farquhar 1967; J. P. Revel & H. J. -P. Ryser 1967, unpublished data; Ryser, Caulfield & Aub 1962). It can be assumed, therefore, that the intracellular degradation of these proteins takes place in these structures.

Uptake of protein by mammalian cells: an underdeveloped area. The penetration of foreign proteins into mammalian cells can be measured and their functions explored

Although it is accepted on the basis of biological and morphological evidence that mammalian cells will take up macromolecules, little is known about the kinetics, the specificity, and the functions of this uptake. With labeled proteins used as models, it is found that the transport proceeds at very low rates, requires little energy, and is markedly enhanced by polybasic compounds. Molecular charge and size are important factors: cells clearly favor cationic macromolecules of large molecular weights. Neither factor, however, can fully account for the selectivity detected in the uptake of different proteins. Ingested albumin undergoes rapid and extensive degradation. This fact suggests that macromolecules have only a limited chance to express their biological activity in target cells, a finding that is relevant also to the role of foreign nucleic acids and the possibility of achieving genetic transformation in animal cells. There are concrete indications, however, that in spite of their short half-life, proteins can act as carriers, as precursors of active agents, and as regulators of metabolic functions in host cells. They may also be important in the control of growth and differentiation. These functions of exogenous proteins are still largely unexplored.