PROTEIN SYNTHESIS, STORAGE, AND DISCHARGE IN THE PANCREATIC EXOCRINE CELL. AN AUTORADIOGRAPHIC STUDY

The synthesis, intracellular transport, storage, and discharge of secretory proteins in and from the pancreatic exocrine cell of the guinea pig were studied by light- and electron microscopical autoradiography using DL-leucine-4,5-H³ as label. Control experiments were carried out to determine: (a) the length of the label pulse in the blood and tissue after intravenous injections of leucine-H³; (b) the amount and nature of label lost during tissue fixation, dehydration, and embedding. The results indicate that leucine-H³ can be used as a label for newly synthesized secretory proteins and as a tracer for their intracellular movements. The autoradiographic observations show that, at ∼5 minutes after injection, the label is localized mostly in cell regions occupied by rough surfaced elements of the endoplasmic reticulum; at ∼20 minutes, it appears in elements of the Golgi complex; and after 1 hour, in zymogen granules. The evidence conclusively shows that the zymogen granules are formed in the Golgi region by a progressive concentration of secretory products within large condensing vacuoles. The findings are compatible with an early transfer of label from the rough surfaced endoplasmic reticulum to the Golgi complex, and suggest the existence of two distinct steps in the transit of secretory proteins through the latter. The first is connected with small, smooth surfaced vesicles situated at the periphery of the complex, and the second with centrally located condensing vacuoles.

HYDROXYUREA: INHIBITORY EFFECT ON DNA METABOLISM

Hydroxyurea, hydroxyurethane, and dihydroxyurea inhibit incorporation of thymidine into the DNA of monolayers of HeLa cells. They do not affect incorporation of uridine into RNA or of leucine into protein. In contrast, hydroxylamine inhibits cellular incorporation of all three precursors: thymidine, uridine, and leucine. Hydroxyurea does not affect thymidine kinase, thymidylate kinase, or DNA polymerase reactions, but it does inhibit incorporation of cytidylic and guanylic acids into DNA in cell-free supernatants.

THE ISOLATION AND PROPERTIES OF CARDIAC RIBOSOMES AND POLYSOMES

1. A method is described by which good yields of ribosomes and polysomes free of contamination by submitochondrial fragments can be prepared from rat cardiac muscle. These preparations are capable of incorporation of amino acids into protein in vitro. 2. The ribosome preparation consists of 32% of monomeric ribosomes and 68% of ribosomal aggregates or polysomes. The polysome preparation has a decreased monomeric content. Dimers, trimers, tetramers, pentamers and larger components can be differentiated. 3. The polysome aggregate structure is degraded to monomeric ribosomes on incubation with small amounts of ribonuclease or by preparation in the absence of Mg(2+) ions. The degradation in the absence of Mg(2+) ions was not reversible and drastically decreased the incorporation of amino acids in vitro. 4. The cardiac ribosomes contained two major RNA species sedimenting at 19s and 28s in a 1:2.4 ratio. 5. The RNA/protein ratio of cardiac ribosomes and polysomes was consistently lower than that of similar preparations from liver. The concentrations of Na(+) and K(+) ions present during preparation had a great effect on the RNA/protein ratio. 6. Optimum conditions for the incorporation of amino acids into protein in vitro are reported. Cardiac ribosomes have a lower rate of incorporation of amino acids in vitro than liver ribosomes. 7. Heart cell sap is less active than liver cell sap: evidence is presented that a factor, present in liver cell sap and concerned with stimulating the synthesis of the peptide chain, is lacking in heart cell sap. 8. Pulse-labelling of perfused hearts followed by examination of the subcellular structures showed that the ribosomal fraction was the most active in the incorporation of amino acids in vitro.

SMALL-SCALE ISOLATION OF FERRITIN FOR THE ASSAY OF THE INCORPORATION OF 14C-LABELLED AMINO ACIDS

1. A new procedure is described for the isolation of pure ferritin from small amounts of tissue. After the removal of most of the tissue proteins by heat coagulation, the ferritin fraction was chromatographed successively on CM-cellulose and Sephadex G-200. 2. The isolated ferritin appeared to be free from other proteins, as judged by its sedimentation pattern in the ultracentrifuge, by electrophoresis on polyacrylamide gel and by immunoelectrophoresis. 3. After the injection of [(14)C]leucine into a series of rats, the specific activity of liver ferritin isolated by the new procedure bore a constant relationship to that of liver ferritin separated by antigen-antibody precipitation. The procedure can thus be used to obtain ferritin of suitable purity for studies of amino acid incorporation. 4. The new procedure can be used to measure the total ferritin protein content of a tissue. It is not possible to use ultraviolet absorption for the measurement of ferritin protein because of considerable interference from the iron that it contains.

METHODS FOR THE PURIFICATION OF THYMUS NUCLEI AND THEIR APPLICATION TO STUDIES OF NUCLEAR PROTEIN SYNTHESIS

Procedures are described for the purification of calf thymus nuclei using mild hypotonit shock to break intact cells, and layering techniques to remove cytoplasmic debris. Ficolc (a high polymer of sucrose) was dissolved in isotonic sucrose to give dense solutions suitable for gradient centrifugation. The method yields nuclei which can incorporate amino acids in vitro. Thymus nuclei isolated under isotonic conditions were incubated with C¹⁴-amino acids and later purified by centrifugation through dense sucrose solutions. The distribution of radioactivity in different nuclear proteins was measured and it was found that isotopic amino acids are actively incorporated into characteristically chromosomal proteins, such as the arginine-rich and lysine-rich histones. Protein synthesis in the nucleus is markedly inhibited by puromycin and by agents, such as 2,4-dinitrophenol, which inhibit ATP synthesis. The synthesis of histones is also inhibited by puromycin, but the uptake of several amino acids into the lysine-rich histone fraction seems less sensitive to puromycin inhibition than is uptake into the arginine-rich histones or other proteins of the nucleus. High resolution autoradiography using tritiated leucine and observing grain distribution over thin sections of isolated nuclei and whole cells shows that amino acid incorporation occurs within the nucleus and is not due to cytoplasmic contamination.

THE STIMULATION BY TREATMENT IN VIVO WITH TRI-IODOTHYRONINE OF AMINO ACID INCORPORATION INTO PROTEIN BY ISOLATED RAT-LIVER MITOCHONDRIA

1. Normal and thyroidectomized rats were treated with near-physiological doses of tri-iodothyronine. Liver mitochondria were isolated and incubated with radioactive amino acids. In normal rats tri-iodothyronine caused only a slight stimulation of incorporation into mitochondrial protein, but in thyroidectomized animals the incorporation was doubled. 2. There was a lag period of about 36 hr. after injection and the maximum effect was observed after 2 days. 3. Direct addition of tri-iodothyronine to the incubation medium had no effect on mitochondrial incorporation. 4. The incorporation was not due to bacterial, nuclear, lysosomal or microsomal contamination and the labelled particles had sedimentation properties identical with those of mitochondria, as followed by suitable enzyme markers. 5. Thyroid hormone treatment did not cause any marked alterations in the pattern of labelling of submitochondrial fractions and in all cases the most radioactive protein was in an insoluble lipid-rich fraction. The amino acid compositions of the total mitochondrial protein and the more radioactive lipoprotein were also unaltered. 6. Increases in the content of RNA and various cytochromes per mg. of mitochondrial protein were observed after treatment with tri-iodothyronine. These occurred slightly later than the stimulation of amino acid incorporation. 7. No uncoupling of oxidative phosphorylation was observed and the ATP production per mg. of mitochondrial protein increased. 8. It was concluded that tri-iodothyronine stimulated amino acid incorporation into mitochondrial protein and that the result is consistent with the view that treatment with thyroid hormone results in an enhanced selective synthesis of mitochondrial respiratory units.

Synthesis and migration of proteins in the cells of the exocrine pancreas as revealed by specific activity determination from radioautographs

Radioautographs of pancreatic acinar cells were prepared in rats and mice sacrificed at various times after injection of leucine-, glycine-, or methionine-H³. Measurements of radioactivity concentration (number of silver grains per unit area) and relative protein concentration (by microspectrophotometry of Millon-treated sections) yielded the mean specific activity of proteins in various regions of the acinar cells. The 2 to 5 minute radioautographs as well as the specific activity time curves demonstrate protein synthesis in ergastoplasm. From there, most newly synthesized proteins migrate to and accumulate in the Golgi zone. Then they spread to the whole zymogen region and, finally, enter the excretory ducts. An attempt at estimating turnover times indicated that two classes of proteins are synthesized in the ergastoplasm: "sedentary" with a slow turnover (62.5 hours) and "exportable" with rapid turnover (4.7 minutes). It is estimated that the exportable proteins spend approximately 11.7 minutes in the Golgi zone where they are built up into zymogen granules, and thereafter 36.0 minutes as fully formed zymogen granules, before they are released outside the acinar cell as pancreatic secretion. The mean life span of a zymogen granule in the cell is estimated to be 47.7 minutes.

LONG-LIVED MESSENGER RNA: EVIDENCE FROM COTTON SEED GERMINATION

In germinating cotton embryos the partial inhibition of RNA synthesis by actinomycin D does not inhibit the incorporation of leucine-(14)C into soluble protein nor cause a loss of polyribosomes during the first 16 hours of germination. This suggests that the protein synthesis observed during this period is directed by messenger RNA which exists in the mature seed and which is not rapidly degraded and resynthesized.

STUDIES ON THE MODE OF ACTION OF DIPHTHERIA TOXIN. II. EFFECT OF TOXIN ON AMINO ACID INCORPORATION IN CELL-FREE SYSTEMS

It has been demonstrated that low concentrations of highly purified diphtheria toxin specifically inhibit incorporation of labeled amino acids into polypeptides in extracts from HeLa cells and from rabbit reticulocytes. No inhibition of incorporation occurs in the absence of a specific cofactor. This cofactor has been identified as nicotinamide adenine dinucleotide (NAD). It has been shown that it is one of the steps involving transfer of amino acids from soluble ribonucleic acid to the growing polypeptide chain that is affected by the toxin in the presence of NAD.

STIMULATORY ACTION OF INSULIN ON LEUCINE UPTAKE AND METABOLISM IN ADIPOSE TISSUE

Epididymal fat incubated in the presence of glucose and dl-leucine-2-C14 incorporated C14 into fatty acids and CO2. Insulin at physiological concentrations increased leucine uptake and conversion to fatty acids and usually decreased oxidation to C14O2. In the absence of glucose, lipogenesis was sharply curtailed, but oxidation of leucine to C14O2 was unchanged. In this situation, insulin failed to increase lipogenesis from leucine, although its stimulatory action on leucine uptake remained. Virtually all the leucine C14 taken up under the influence of insulin now appeared as C14O2. Insulin similarly increased the uptake and conversion of acetate-2-C14 to fatty acids and decreased C14O2 production in the presence of glucose but not in its absence. Addition of α-aminoisobutyric acid to the medium blocked the stimulatory effect of insulin on leucine uptake. From these observations it is suggested that insulin may affect the synthesis of fatty acids from leucine in two ways: directly by increasing the transport of leucine into the cell and indirectly by increasing the availability of glucose and hence channeling the acetate derived from leucine into fatty acids.

GENETIC CONTROL OF RESTRICTION AND MODIFICATION IN ESCHERICHIA COLI

Boyer, Herbert (Yale University, New Haven, Conn.). Genetic control of restriction and modification in Escherichia coli. J. Bacteriol. 88:1652-1660. 1964.-Bacterial crosses with K-12 strains of Escherichia coli as Hfr donors (Hfr Hayes, Hfr Cavalli, and Hfr P4X-6) and B/r strains of E. coli as F(-) recipients were found to differ from crosses between K-12 Hfr donors and K-12 F(-) recipients in two ways: (i) recombinants (leu, pro, lac, and gal) did not appear at discrete time intervals but did appear simultaneously 30 min after matings were initiated, and (ii) the linkage of unselected markers to selected markers was reduced. Integration of a genetic region linked to the threonine locus of K-12 into the B/r genome resulted in a hybrid which no longer gave anomalous results in conjugation experiments. A similar region of the B strain was introduced into the K-12 strain, which then behaved as a typical B F(-) recipient. These observations are interpreted as the manifestation of host-controlled modification and restriction on the E. coli chromosome. This was verified by experiments on the restriction and modification of the bacteriophage lambda, F-lac, F-gal, and sex-factor, F(1). It was found that the genetic region that controlled the mating responses of the K-12 and B/r strains also controlled the modification and restriction properties of these two strains. The genes responsible for the restricting and modifying properties of the K-12 and B strains of E. coli were found to be allelic, linked to each other, and linked to the threonine locus.