Structure and function of aspartate transcarbamoylase studied using chymotrypsin as a probe.

Aspartate transcarbamoylase from Escherichia coli is composed of six catalytic (c) and six regulatory (r) polypeptides. We have studied the structure and function of this enzyme using chymotrypsin as a probe. The protease inactivates the isolated catalytic subunit (c3) but has not effects on the native enzyme (c6r6). Under identical conditions, the c3r6 complex is inactivated at a much slower rate than c3. The presence of the substrate analogue succinate together with carbamoyl phosphate reduces substantially the rate of inactivation. Extended exposure to chymotrypsin converts the catalytic subunit into a partially active derivative with a fourfold higher Michaelis constant. This derivative is indistinguishable from the unmodified catalytic subnit in gell electrophoresis under nondenaturing conditions. However, in the presence of sodium dodecyl sulfate, the major fragment in the electropherogram is smaller than that of the intact catalytic polypeptide. The results could be explained by postulating the presence of a chymotrypsin-sensitive peptide bond at or near the active site. Since X-ray crystallographic studies have indicated that the active sites are located in a central cavity, the resistance of the native enzyme towards inactivation may be due to the inability of chymotrypsin to enter this cavity.     

Histidine:pyruvate transamination and phyenylalanine:pyruvate transamination may be catalyzed by the same enzyme.

Some properties of histidine:pyruvate transaminase (HPT) and phenylalanine:pyruvate transaminase (PPT) in the cytosol of rat liver were studied. HPT and PPT activity could not be separated by DEAE-Sephadex A-50 or hydroxylapatite column chromatography, and the ratio of $\text{HPT}\text{PPT}$ activity remained constant during these purification procedures. The two enzyme activities also showed similar heat stability and responses to glucagon injection. Based on these findings, we suggest that a single enzyme may specifically catalyze histidine:pyruvate and phenylalanine:pyruvate transamination.     

Isolation and characterization of the major apolipoprotein from chicken high density lipoproteins.

High density lipoproteins were isolated from plasma of white Leghorn hens by ultracentrifugal flotation between densities 1.063 and 1.210 g/ml. After delipidation, the lipid-free proteins were fractionated by chromatography on Sephadex G-150 in urea; one major apolipoprotein was isolated and characterized. From its chemical, physical and immunochemical properties, the major apoprotein from hen high-density lipoproteins has characteristics similar to the major apoprotein of human high density lipoproteins, apoA-I. Thus the hen protein has been designated hen apoA-I. Hen apoA-I has a molecular weight of approximately 28 000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Its calculated molecular weight from its 234 constituent amino acids is 26 674. Hen apoA-I differed from its human counterpart by containing isoleucine. Treatment of hen apoA-I with carboxypeptidase A yielded a COOH-terminal sequence of Leu-Val-Ala-Gln. Automatic Edman degradation of the apoprotein gave an NH2-terminal sequence of Asp-Glu-Pro-Gln-Pro-Glu-Leu. Hen apoA-I had a circular dichroic spectrum typical of alpha-helical structures; the calculated helicity was 90%. Goat antisera prepared to hen apoA-I formed precipitin lines of complete identity to the hen apoprotein but lines of only partial identity to human apoA-I. These studies show that the major apoprotein from hen and human high-density lipoproteins have similar properties to each other suggesting a common physiologic function.     

Purification and properties of ATPase inhibitor from rat liver mitochondria

(1) The ATPase inhibitor protein has been isolated from rat liver mitochondria in purified form. The molecular weight determined by sodium dodecyl sulfate gel electrophoresis is approximately 9500, and the isoelectric point is 8.9.

(2) The protein inhibits both the soluble ATPase and the particle-bound ATPase from rat liver mitochondria. It also inhibits ATPase activities of soluble F₁, and inhibitor-depleted submitochondrial particles derived from bovine heart mitochondria.

(3) On particle-bound ATPase the inhibitor has its maximal effect if incubated in the presence of Mg²⁺. ATP at slightly acidic pH.

(4) The inhibitor has a minimal effect on P_i-ATP exchange activity in sonicated submitochondrial particles. However, unexpectedly the inhibitor greatly stimulates P_i-ATP exchange activity in whole mitochondria while the low ATPase activity of the mitochondria is not affected. The possible mechanism of action of the inhibitor on intact mitochondria is offered.     

Proton magnetic resonance studies of lipid bilayer membranes Experimental determination of inter- and intramolecular nuclear relaxation rates in sonicated phosphatidylcholine bilayer vesicles

We have determined the relative magnitudes of the intra- and intermolecular contributions to the nuclear magnetic relaxation rates of the methylene protons of the hydrocarbon chains in phosphatidylcholine bilayer vesicles over a range of temperatures and at two NMR frequencies (100 and 220 MHz). These measurements have been made by the isotopic dilution method using deuterated phosphatidylcholines containing fully deuterated hydrocarbon chains. The results showed that both the methylene linewidths and the spin-lattice relaxation rates are dominated by intramolecular dipolar interactions. Both the intra- and intermolecular contributions to the spin-lattice relaxation rate were found to decrease with increasing temperature and to exhibit a frequency dependence, the rates being higher at the lower NMR frequency in both cases. These observations indicate that both intra- and intermolecular dipolar interactions are modulated by anisotropic motions. In the case of the intermolecular dipolar fields, it is proposed that they are modulated both by the rapid rotational isomerization of the hydrocarbon chains as well as by lateral diffusion of the lipid molecules. That the hydrocarbon chain motion must be fairly effective in effecting efficient spin-lattice relaxation is evident from the negligible intramolecular interchain contribution to the relaxation found in the present work.     

Linear structure of the oligosaccharide chains in alpha1-protease inhibitor isolated from human plasma.

Two glycopeptides present in equal amounts were isolated from a pronase digest of alpha1-protease inhibitor of human plasma by gel filtration on Sephadex G-50 and chromatography on DEAE-cellulose. The carbohydrate side chains in both glycopeptides are linked through asparaginyl residues. The glycopeptides were digested sequentially with specific glycosidases; and after each step, the released sugars as well as the composition of the residual peptides were determined. The linear structures of these glycopeptides deduced from these data are shown below. Based on the total carbohydrate content of the intact protein and with these structural data, it is postulated that 4 oligosaccharide units are attached to 1 molecule of the protein; 2 of these were represented as in Equation 1, the other 2 as in Equation 2.     

Adrenal inhibition of maternal behavior in virgin female rats.

The role of the adrenal gland in suppressing the onset of maternal behavior in virgin female rats was investigated. Virgin female rats were either ovariectomized, adrenalectomized, or subjected to the combined operation and tested for the induction of maternal behavior by being exposed to young pups. Females that were both ovariectomized and adrenalectomized 4 weeks prior to testing exhibited significantly shorter latencies to the onset of maternal behavior than that of females subjected to either ovariectomy or adrenalectomy alone. Replacement of either estrogen or progesterone in the group that was both adrenalectomized and ovariectomized resulted in a prolongation of the average latency to become maternal. The results indicated that both estrogen and progesterone can act to inhibit the onset of maternal behavior and that the adrenal gland may act to suppress the onset of such behavior by supplying an extra-ovarian source of endogenous progesterone and estrogen.     

Regulation of nitrogen fixation. Nitrogenase-derepressed mutants of Klebsiella pneumoniae.

1. A new procedure is described for selecting nitrogenase-derepressed mutants based on the method of Brenchley et al. (Brenchley, J.E., Prival, M.J. and Magasanik, B. (1973) J. Biol. Chem. 248, 6122-6128) for isolating histidase-constitutive mutants of a non-N2-fixing bacterium. 2. Nitrogenase levels of the new mutants in the presence of NH4+ were as high as 100% of the nitrogenase activity detected in the absence of NH4+. 3. Biochemical characterization of these nitrogen fixation (nif) derepressed mutants reveals that they fall into three classes. Three mutants (strains SK-24, 28 and 29), requiring glutamate for growth, synthesize nitrogenase and glutamine synthetase constitutively (in the presence of NH4+). A second class of mutants (strains SK-27 and 37) requiring glutamine for growth produces derepressed levels of nitrogenase activity and synthesized catalytically inactive glutamine synthetase protein, as determined immunologically. A third class of glutamine-requiring, nitrogenase-derepressed mutants (strain SK-25 and 26) synthesizes neither a catalytically active glutamine synthetase enzyme nor an immunologically cross-reactive glutamine synthetase protein. 4. F-prime complementation analysis reveals that the mutant strains SK-25, 26, 27, 37 map in a segment of the Klebsiella chromosome corresponding to the region coding for glutamine synthetase. Since the mutant strains SK-27 and SK-37 produce inactive glutamine synthetase protein, it is concluded that these mutations map within the glutamine synthetase structural gene.