Pancreas, amino acid incorporation

In the second phase of these investigations on protein synthesis, an attempt was made to study the process in vitro. At first, intact cells had to be used. Amino acid incorporation was studied in liver slices amylase, lipase, and ribonuclease synthesis was investigated in slices of pigeon pancreas and the incorporation of glycine into reticulocyte hemoglobin was measured. The first of the above systems was exploited with exceptional success in Zamecnik s laboratory [138]. 

Despite earlier suspicions, it is now well established that mitochondria from a variety of sources can incorporate amino acids into protein independently of the microsomal fraction. But, with the exception of muscle tissue (67), the initial rate (1-5 min after injection of the isotope) of amino acid incorporation into mitochondria of animal tissues, in vivo, is much less than that of the microsomal fraction. The ratio of the specific activities of these two fractions, however, varies considerably from tissue to tissue (276)-, for rat liver, for example, it is 5, whereas for guinea pig pancreas it is only 1.8. After longer periods, since the specific activity of the microsomal fraction reaches a saturation value after about 10 min, the specific as well as the total activity of the mitochondria eventually surpasses that of the microsomes in most tissues. In plants, mitochondria seem to be relatively more active than in animals (68, 69). 

Similar in vivo labeling studies contributed other important information they demonstrated that the rate of protein synthesis was different in different tissues. Although the proteins of liver, kidney, intestinal mucosa, spleen, pancreas, bone marrow, and testicles are rapidly labeled, labeling is slow in muscle, brain, erythrocyte, and skin. Within a given tissue, some proteins are labeled faster than others. In the pancreas, the hydrolytic enzymes incorporate amino acids more rapidly than other proteins. The label appears sooner in some cell fractions than in others. After in vivo administration of antigens, 50% of the antibodies can be recovered in the microsomal fraction only a few minutes after injection. It was also shown that under conditions of net protein synthesis, labeled amino acids injected in mice are first recovered in the microsomes, then in the supernatant, and later in the zymogen granules of the pancreas. 

Electron microscopy has revealed the existence of two types of polyribosomes—some bound to the membrane and others free. It is believed that the bound polyribosomes are involved in elaboration of export proteins, while nonbound polyribosomes are involved in the biosynthesis of proteins to be used for local consumption. In the pancreas, it has been shown that chymotrypsin is formed on bound ribosomes. Redman studied the incorporation of labeled amino acid in vivo and in vitro in purified serum proteins and ferritin of rat liver and showed that the radioactivity incorporated in albumin is associated with the bound ribosome, whereas that of ferritin is associated with free ribosomes [220]. 

In an experiment designed to follow the synthesis and secretion of zymogen, a radioactive amino acid was Injected into the pancreas of a guinea pig to label proteins undergoing synthesis (pulse). Since most proteins in pancreas are synthesized as zymogens, the radioactive amino acid would be incorporated primarily in gmiogens. Three minutes after the pulse, all incorporated radioactivity was found to be present in the rough endoplasmic reticulum (ER). 

MWCNT electrodes have been developed to monitor the electrochemical oxidation of insulin, a pancreas-produced hormone that plays a key role in the regulation of carbohydrates and fat metabolism in the body. This provides a possible method to evaluate the quality of pancreatic islets before their transplantation. By coating similar MWCNT electrodes with platinum microparticles, thiols containing amino acids can be detected in rat striatal that is the subcortical part of the fore-brain. CNT-based sensors can be incorporated into flexible biocompatible substrates to facilitate in vivo sensing. For instance, free cholesterol in blood can be measured using MWCNT electrodes placed on a biocompatible substrate whereas flexible pH sensors can be formed from polyaniline (a conductive polymer) and nanotube composites. ... 

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