Biochimica et Biophysica Acta (BBA) - Enzymology最新文献

A chymotrypsin-like enzyme (EC 3.4.21.20) was isolated from bovine granulocytes, and purified 14-fold by affinity chromatography on 4-phenylbutylamine Affi-gel. The molecular weights of the isoenzymes were estimated as 16 000, 19 300 and 24 000 by SDS-polyacrylamide gel electrophoresis. A Michaelis constant of 2 mM and a catalytic constant of 34.6 s⁻¹ were determined with Bz-Tyr-OEt. The esterolytic activity of the enzyme could be inhibited both by PMSF and by TPCK. It was also inhibited by chymostatin ($\text{K}{}_{\mathrm{<mtext>i</mtext>}}\text{= 0.11 μ}\text{g/ml}$) and by the cytosol inhibitor of the bovine granulocyte ($\text{K}{}_{\mathrm{<mtext>i</mtext>}}\text{′ = 1 μ}\text{M}$). The chymotrypsin-like enzyme of the bovine granulocyte shares a number of the kinetic properties common to the chymotrypsin-like enzyme of the human granulocyte. The two granulocytes showed nearly identical chymotrypsin-like enzyme activities per cell.

Two new sites phosphorylated by rat liver cyclic AMP-independent casein kinase TS have been identified in denatured pepsin and soybean antiprotease C-II, exhibiting the sequences: Cys-Ser-Ser(P)-Ile-Asp-Ser and His-Ser₃(P)-Asp-Asp-Glu, respectively. Their phosphorylation efficiency has been compared to that of previously identified sites and the effects of chemical modifications in the vicinity of the phosphorylatable residue have been studied. The results obtained support the following conclusions: 1. All sites affected by casein kinase TS conform to the sequence: Ser/Thr-X-Glu/Asp which is also believed to be required by the mammary gland casein kinase. Threonine appears to be less suitable for phosphorylation than serine. The presence of some additional residues on the C-terminal side also appears to be required. 2. × can be either an additional acidic residue or a neutral one, but not a basic residue. The contiguity of an acidic cluster to the C-terminal side of the target greatly improves the phosphorylation efficiency. 3. The residues N-terminal to the target one do not seem to be relevant for determining the site recognition by the protein kinase. 4. The predicted secondary structure constantly occurring at the phosphorylation sites is the β-turn: apparently the bend must include both the target residue and the acidic determinant at the $\text{n+ 2}$ position.

Nine synthetic peptides reproducing either exactly or with suitable substitutions three phosphorylatable sites of the protamines thynnine Z1 and galline have been prepared and tested as phosphate acceptors for the rat liver cyclic AMP-dependent protein kinase (type I). The most significant results obtained can be summarized as follows: 1. The hexapeptide: Arg-Arg-Ser-Thr-Val-Ala gives two phpsphorylated products, containing only Ser-P and both Ser-P and Thr-P, respectively. The relative amount of the di-phosphorylated derivative is not dependent on the incubation time but rather on the concentration of the substrates. 2. Both the replacements of ornithine for the two N-terminal arginines of glutamic acid for Val₅ in the above peptide completely prevent phosphorylation, without conferring any inhibitory activity to the modified peptides, thus supporting the view that the N-terminal guanido groups and the C-terminal hydrophobic residue(s) are both required for the binding at the catalytic site rather than for the subsequent transphosphorylation reaction. 3. The replacement of alanine for Ser₃ gives rise to a peptide whose Thr₄ residue is still phosphorylated with an efficiency comparable to that of the unmodified peptide. The di-substituted peptide: Arg-Arg-Ala-Ser-Val-Ala however exhibits a dramatically lower $\text{K}{}_{\mathrm{m}}$ value indicating that serine is a much better target than threonine whenever it is not adjacent to the N-terminal arginine couple. 4. The importance of the distance between the target residue and the N-terminal basic determinants is also evidenced by the phosphorylation of the dodecapeptide: Pro-(Arg)₅-Ser-Ser-Arg-Pro-Val-Arg exhibiting a $\text{K}{}_{\mathrm{m}}$ value about 20-times higher than that of salmine A1, whose phosphorylation site is comprised within an identical amino acid sequence, including however three rather than two adjacent serine residues. 5. The tetradecapeptide: (Arg)₄-Tyr-Gly-Ser-(Arg)₆-Tyr is completely unaffected by the kinase though a very similar site is found phosphorylated in native iridines, probably through a cyclic AMP-independent mechanism.

α-N-Acylamino acid hydrolase from bovine liver has been purified approx. 1700-fold with a yield of 28%. Based on the results of studies using polyacrylamide gel electrophoresis the purified enzyme is homogeneous. This enzyme catalyzes the hydrolysis of α-N-acylated amino acids to yield the acyl group and the corresponding amino acid, α-Acetylmethionine was the most rapidly hydrolyzed substrate tested for this enzyme. Most of the other α-N-acylated amino acids used as substrates were hydrolyzed. Some, however, were cleaved at less than 1% of the rate observed for N-acetylmethionine, α-N-Acylamino acid hydrolase did not catalyze the hydrolysis of $\text{N-}\text{acetyl-}\text{d}\text{-alanine}$, N-acetyl-β-alanine, ϵ-N-acetyllysine, N-acetylglycinamide or α-N-acetylated peptides. When incubated with this enzyme the chloroacetyl derivatives of leucine and valine were hydrolyzed approx. 8- and 3-times, respectively, more rapidly than the corresponding N-acetyl derivatives. On the other hand, N-formylmethionine was hydrolyzed at only 60% the rate of N-acetylmethionine. Other studies on α-N-acylamino acid hydrolase have shown that it is a dimer composed of two 40 000 molecular weight monomers, it is active over a broad pH range with maximal activity at approx. pH 8.5, and requires Zn²⁺ for enzymatic activity. The substrate specificity and other properties of bovine liver α-N-acylamino acid hydrolase are very similar to those reported for acylase I isolated from porcine kidney [7]. We suggest that this enzyme functions in the catabolism of α-N-acetylamino acids resulting from the turnover of α-N-acetylated proteins.

CTP synthetase (UTP: ammonia ligase (ADP-forming), EC 6.3.4.2) was purified 200-fold from Ehrlich ascites tumor cells with about 50% purity as judged by polyacrylamide gel electrophoresis. The molecular weight was estimated to be 118 000 by gel filtration. The optimal pH was 8.6. 2-Mercaptoethanol was required for optimal activity and stabilization of the enzyme. Magnesium was essential for the reaction and other divalent cations were ineffective. Ammonia could replace glutamine as the amino donor. When other substrates were at saturating concentrations, Michaelis-Menten kinetics were observed yielding $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for UTP, ATP and glutamine of 0.18, 0.8 and 0.13 mM, respectively. With ATP at subsaturating concentration, the double-reciprocal plot for UTP saturation was sinusoidal and the Hill plot showed an $\text{n}$ value of 1.3. The double-reciprocal plot for ATP saturation, when UTP was at subsaturating concentration, departed from Michaelis-Menten kinetics with an $\text{n}$ value of 1.4. These data suggest the existence of cooperative interactions between enzyme and substrates at subsaturating concentrations of ATP or UTP. GTP was not essential, but it acted as an activator on the glutamine reaction; optimal activation was observed at 1 mM GTP. The affinity for glutamine was not affected by GTP.

$\text{α}{}_1\text{-}\text{Antitrypsin}$ is the second most abundant proteinase inhibitor in plasma. The fact that it is a globular glycoprotein of relatively small size ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 53 500) allows it access to a wide variety of fluids and tissue sites. $\text{α}{}_1\text{-}\text{Antitrypsin}$ has been purified from mouse plasma by affinity chromatography and ion exchange. The purified protein exhibits homogeneity on polyacrylamide electrophoresis, but electrophoretic heterogeneity on crossed immuno-electrophoresis. Mouse and rat $\text{α}{}_1\text{-}\text{antitrypsin}$ show strong crossreactivity and the half-life for mouse $\text{α}{}_1\text{-}\text{antitrypsin}$ is 15.5 h. Fetal levels are 15% of adult and it requires 25–30 days before adult levels are reached in the neonate. Maternal levels remain unchanged throughout pregnancy and at parturition. The inhibitor is present in a number of body fluids including serum, breast milk, gastrointestinal washings, lung washings and bile. The source of $\text{α}{}_1\text{-}\text{antitrypsin}$ for all of these fluids appears to be the liver.

The presence and the localization of the enzyme catalyzing the transfer of a coenzyme A molecule from succinylCoA to 3-hydroxy-3-methylglutarate has been established in rat liver mitochondria. The enzyme was found mainly in the mitochondrial matrix but some activity was also found in the inner membrane fraction. The enzyme has been purified about 100-fold from sonically-disrupted mitochondria by high-speed centrifugation, DEAE-cellulose chromatography, (NH₄)₂SO₄ precipitation and Sephadex G-100 filtration. The enzymatic activity was recovered in the final step as a single peak. The coenzyme A transferase appears to have a molecular increases the activity and improves its stability. The enzyme is different from the succinylCoA: 3-oxoacids coenzyme A transferase and is active also on malonylCoA. The apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values obtained for succinylCoA, malnyCoA and 3-hydroxy-3-methylglutarate were 2.2 · 10⁻⁴ M, 3.7 · 10⁻⁴ M and 1.7 · 10⁻³ M, respectively. Acetoacetate, which is the final product of the mitochondrial metabolism of hydroxy-methylglutarylCoA, showed an inhibitory effect on the enzyme activity with a $\text{K}{}_{\mathrm{i}}$ of 0.5 mM. The physiological role of the enzyme is discussed.

Talaromyces emersonii, a thermophyllic fungus was grown on cellulose/corn steep liquor/NH₄NO₃ medium. The kinetics of growth and extracellular cellulase production were measured. The enzyme system was found to be comprised of four to five forms of exo-β-1,4-glucanase (cellobiohydrolase; 1,4-β-d-glucan cellobiohydrolase, EC 3.2.1.91), at least two forms of endo-1,4-β-glucanase (1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4) and three enzymes exhibiting β-glucosidase (cellobiase; β-d-glucoside glucohydrolase, EC 3.2.1.21) activity. One of the latter, termed β-glucosidase III, is induced concurrently with the cellulases but disappears from the medium because of the low pH that develops during growth. The cellulases by contrast are more acid-stable. Later in the growth cycle a second from of β-glucosidase, termed β-glucosidase I, accumulates. An intracellular β-glucosidase, β-glucosidase IV, was also detected. The possible functions of these enzymes are discussed.

The thermophyllic fungus Talaromyces emersonii produces three extracellular and one intracellular enzymes exhibiting β-glucosidase (1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.21) activity. Two of the extracellular forms β-glucosidase I and β-glucosidase III have been purified as has the intracellular, β-glucosidase IV. The pH and temperature, optima, stability, kinetic parameters and substrate specificity of each has been determined. We conclude that β-glucosidase I and β-glucosidase IV are true cellobiases while β-glucosidase III is an exo-β-1,4-glucose hydrolase.