Electron microscopy of enzymes

Messenger RNA for glutamine synthetase. Review article

Of the various eucaryotic tissues, where glutamine synthetase (GS) mRNA and its regulation have been investigated, the induction of GS by glucocorticoids in the embryonic chick retina represents one of the systems most extensively studied. GS mRNA was first identified at the polysomal level by immunochemical precipitation of fractionated polysomes containing nascent GS chains with anti-GS gamma-globulin. The mRNA has been shown to be polyadenylated at the 3' end; on this basis, it has been partially purified from embryonic chick retina as well as from N. Crassa by chromatography on oligo(dT)-cellulose or poly(U)-sepharose and translated in cell-free protein synthesizing systems derived from wheat germ. Hormonal regulation of GS activity studied in the embryonic retina, hepatoma tissue culture cells, or in other tissues is always shown to be mediated by GS mRNA. In the retina, hydrocortisone (HC) elicits an age-related and transcription-dependent induction of GS by enhancing the level of GS mRNA in the polysomes through an increased supply of this mRNA from the nucleus. Comparative studies of three inhibitors of transcription, viz. actinomycin D, leucanthone and proflavine on the induction of GS by HC indicate that the latter inhibits GS mRNA selectively and reversibly with minimal effects on other RNA synthesis. Since proflavine acts by competing with HC-receptor binding sites in the nuclei, further studies on its interaction with the retina genome are likely to help identify the DNA sequences involved in the GS induction. In bacteria, studies on the genetics and physiology of various mutants with lesions in the structural gene for GS show that the transcription of the GS gene (gln A) is regulated both positively and negatively by GS and the product of another gene gln F. Purification of GS mRNA to homogeneity cloning of its cDNA and development of assay systems for cell-free transcription of GS are other studies likely to advance our knowledge on GS mRNA and its regulation.

Permeability of muscle capillaries to exogenous myoglobin

Whale skeletal muscle myoglobin (mol wt 17,800; molecular dimensions 25 x 34 x 42 A) was used as a probe molecule for the pore systems of muscle capillaries. Diaphragms of Wistar-Furth rats were fixed in situ at intervals up to 4 h after the intravenous injection of the tracer, and myoglobin was localized in the tissue by a peroxidase reaction. Gel filtration of plasma samples proved that myoglobin molecules remained in circulation in native monomeric form. At 30-35 s postinjection, the tracer marked approximately 75% of the plasmalemmal vesicles on the blood front of the endothelium, 15% of those located inside and none of those on the tissue front. At 45 s, the labeling of vesicles in the inner group reached 60% but remained nil for those on the tissue front. Marked vesicles appeared on the latter past 45 s and their frequency increased to approximately 80% by 60-75 s, concomitantly with the appearance of myoglobin in the pericapillary spaces. Significant regional heterogeneity in initial labeling was found in the different segments of the endothelium (i.e., perinuclear cytoplasm, organelle region, cell periphery, and parajunctional zone). Up to 60 s, the intercellular junctions and spaces of the endothelium were free of myoglobin reaction product; thereafter, the latter was detected in the distal part of the intercellular spaces in concentration generally equal to or lower than that prevailing in the adjacent pericapillary space. The findings indicate that myoglobin molecules cross the endothelium of muscle capillaries primarily via plasmalemmal vesicles. Since a molecule of this size is supposed to exit through both pore systems, our results confirm the earlier conclusion that the plasmalemmal vesicles represent the large pore system; in addition, they suggest that the same structures are, at least in part, the structural equivalent of the small pore system of this type of capillaries.

Regulation of rat liver glutamine synthetase: activation by alpha-ketoglutarate and inhibition by glycine, alanine, and carbamyl phosphate

Rat liver glutamine synthetase (s(20,w) 15.0 S; MW about 352,000) resembles ovine brain glutamine synthetase in that it (a) has 8 subunits, (b) acts on both L- and D-glutamate and certain glutamate analogs (e.g., beta-glutamate, cis-cycloglutamate, and alpha-methyl-L-glutamate), and (c) is irreversibly inhibited by L-methionine-S-sulfoximine. The liver enzyme (but not the brain enzyme) is (a) markedly activated by alpha-ketoglutarate and less so by citrate, and (b) inhibited noncumulatively by glycine and alanine, in the presence of Mn(++) but not Mg(++); inhibition increases with increasing concentrations of glutamate. These regulatory phenomena seem to be correlated with metabolically related enzymes, e.g., glutamine transaminase. Both liver and brain glutamine synthetases are inhibited by carbamyl phosphate (with Mn(++) but not with Mg(++)), which provides a means for controlling glutamine for pyrimidine biosynthesis. Addition of Mn(++) to the Mg(++)-synthetase system, even at Mg(++): Mn(++) ratios of 1000, markedly inhibits synthesis by both brain and liver enzymes. This finding, and the fact that Mn(++) promotes sensitivity to the negative effectors, indicates that Mn(++) plays a central role in the regulation of glutamine synthetase. Properties of the glutamine synthetases that have been isolated from mammalian, plant, and bacterial cells are compared. They are similar with respect to subunit size, substrate specificity, inhibition by methionine sulfoximine, and Mn(++)-sensitive inhibition by glycine, alanine, and carbamyl phosphate, but differ in certain other regulatory phenomena and in subunit structure.