Membrane transport of proteins

The incoherence of the vesicle theory of protein secretion

The rates at which cells secrete peptides and proteins must on average equal their rate of synthesis. This basic equality has unanticipated and seemingly categorical negative consequences for the vesicle theory of protein secretion. This is because the transport mechanisms it proposes, such as the budding and fusion of small vesicles and secretion by exocytosis, are not capable of balancing forces. What follows is an account of the analysis that leads to this conclusion.

Conservation of digestive enzymes

The traditional understanding is that an entirely new complement of digestive enzymes is secreted by the pancreas into the small intestines with each meal. This is thought to be necessary because, like food itself, these enzymes are degraded during digestion. In this review we discuss experiments that bring this point of view into question. They suggest that digestive enzymes can be absorbed into blood, reaccumulated by the pancreas, and reutilized, instead of being reduced to their constituent amino acids in the intestines. This is called an enteropancreatic circulation of digestive enzymes.

Equilibration of pancratic digestive enzymes across zymogen granule membranes

Zymogen granules isolated from the rat pancreas released digestive enzyms in response to lowering the enzyme concentration of the medium in which the granules were suspended, and reabsorbed enzyme when the enzyme concentration of the medium was increased. The evidence suggests that the distribution of digestive enzyme between zymogen granule and medium is the result of an equilibrium process. This equilibrium is apparently expressed across the granule membrane since the disruption of the membrane barrier eliminated the delay in establishing new equilibrium between the bound and free enzyme. The release of enzyme was continuous (equilibration did not occur) when the suspending medium was continuously filtered, thereby maintaining a constant state of disequilibrium between intragranular enzyme and enzyme in the medium. The data suggest the following: (1) that digestive enzyme passes through the intact granule membrane bidirectionally, and (2) that there is a free cytoplasmic pool of digestive enzyme in intact cells which is in equilibrium with the enzyme content of the zymogen granule.

Transport of amylase across the apical membrane of the pancreatic exocrine cells. Quantitative analysis of zymogen granules

The number or the size of the zymogen granules was calculated in a determined volume slice of pigeon pancreas and in a total cellular fraction of zymogen granules isolated from a part of this organ having the same volume. The obtained values were not significantly different. These results are discussed in relation to the mechanism of digestive enzyme secretion. They confirm that amylase can be transported molecule by molecule across the intact apical membrane of the pancreatic exocrine cells.

Interaction of cultured mammalian cells with [125I] diphtheria toxin

The characteristics of cell adsorption and pinocytotic uptake of diphtheria toxin by several mammalian cell types were studied. Purified toxin iodinated by a solid-state lactoperoxidase method provided preparations of high specific activity and unaltered biological activity. Dephtheria toxin-sensitive HEp-2 cells and guinea pig macrophage cultures were compared with resistant mouse L-929 cells. At 37 C the resistant cells in monolayer adsorbed and internalized [125I] toxin to a greater extent than did the HEp-2 cell cultures; no significant differences were observed at 5 C. Ammonium chloride protection levels did not alter uptake of toxin by either L-929 OR HEp-2 cells. Biological activity of the iodinated toxin, however, was negated provided the presence of ammonium chloride was maintained. The ammonium salt appears to maintain toxin in a state amenable to antitoxin neutralization. Guinea pig macrophages internalized iodinated toxin to a level 10 times greater than the established cell lines. In spite of the increased uptake of toxin by the endocytic cells, ammonium chloride prevented expression of toxicity. In an artificial system, toxin adsorbed to polystyrene latex spheres and internalized by guinea pig macrophages during phagocytosis did express biological activity. Ammonium chloride afforded some but not total protection against toxin present in the phagocytic vacuoles. The data suggest that two mechanisms of toxin uptake by susceptible cells may be operative. Toxin taken into the cell by a pinocytotic process probably is not ordinarily of physiological significance since it is usually degraded by lysosomal enzymes before it can reach cytoplasmic constituents on which it acts. When large quantities of toxin are pinocytized, toxicity may be expressed before enzymatic degradation is complete. A more specific uptake involving direct passage of the toxin through the plasma membrane may be the mechanism leading to cell death in the majority of instances.

Studies on the release of barley aleurone cell proteins: Kinetics of labelling

Protein release from gibberellic acid-treated aleurone cells of barley (Hordeum vulgare L.) was followed in pulse-chase experiments with radioactively labelled amino acids. After a 10-min pulse of [(3)H]leucine or [(3)H]tryptophan, label was incorporated into trichloroacetic-acid (TCA)-insoluble material; some of this was released into the incubation medium during a chase with carrier amino acid. This relase of TCA-insoluble material into the incubation medium had no appreciable lag period. Precipitation with rabbit-anti-α-amylase antibody of the radioactivity released from aleurone layers into the medium during chasing indicates that as much as 70% of the radioactivity present is α-amylase. Aleurone cell homogenates were fractionated by differential centrifugation after pulsing with labelled amino acids. Radioactivity in TCA-insoluble materials was distributed equally among all sediment fractions indicating that no specific accumulation of label occurred. Tissue was also fractionated after labelling with a pulse of [(3)H]tyrosine and [(14)C]-tryptophan, and the distribution of radioactivity in various fractions also showed that no preferential sedimentation of label occurred. Altogether, no experimental evidence could be found to support the hypothesis that proteins are released from aleurone cells via discrete secretory organelles.