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E Grazi

Publications and source records attributed to E Grazi.

At least 73 records · Page 4Linked to original sources
PubMed1982

Multiple supramolecular structures formed by interaction of actin with protamine.

When protamine is added to actin, different supramolecular structures are formed depending on the molar ratio of the two proteins and of the ionic strength of the medium. At low ionic strength, and going from a molar ratio of protamine to G-actin of 4:1, 2:1 and 1:1, globular aggregates are first converted into extended structures and then to long threads in which the constituent ATP-G-actin is rapidly exchangeable with the actin of the medium. At high ionic strength {Tyrode [(1910) Arch. Int. Pharmacodyn. Ther.20, 205-212] solution}, starting from G-actin and protamine in the 1:1 molar ratio, long ropes are formed that can be resolved into intertwining filaments of 4-5nm diameter. The addition of protamine in a 1:1 molar ratio to a solution of F-actin in Tyrode solution causes the breakage of the actin filaments, which is also revealed by the decrease of the viscosity of the solution and the formation of ordered latero-lateral aggregates. The structures formed by reaction of protamine with G-actin can be separated from free G-actin and protamine by filtration through 0.45mum-pore-size Millipore filters. This technique has been exploited to study the exchange reaction between free actin and the actin-protamine complexes. For these studies the 1:1 actin-protamine complex formed at low ionic strength and the 2:1 actin-protamine complex formed in the presence of 23nm-free Mg(2+) have been selected. In the first case the exchange reaction is practically complete in the dead time of the experiment (20s). In the second case, where the complex operates like a true ATPase, the rate of the exchange is initially comparable with the rate of the ATP cleavage. Later on, however, the complex undergoes a change and the rate of the exchange between free actin and the actin bound to protamine becomes lower than the rate of the ATPase reaction. It is proposed that the ATP exchanges for ADP directly on the G-actin bound in the complex.

Actins↗
PubMed1981

Different polymeric forms of actin detected by the fluorescent probe terbium ion.

The interaction of actin with Tb3+ was studied by following the fluorescence emitted at 545 nm when the protein was excited at 285 nm in the presence of Tb3+. It was shown that, at low ionic strength, each actin monomer binds, at saturation, six Tb3+ with an association constant of 0.8 microM-1. In the presence of 0.1 M KCl the association constant decreases to 0.15 and 0.24 microM-1 at subcritical and overcritical actin concentrations, respectively; the number of the binding sites remains six. When polymeric actin is formed by the addition of 2 mM MgCl2, the association constant drops to 0.008 micro M-1 and the number of the binding sites to four. The lower number of the Tb3+ binding sites (four) in the actin polymerized by MgCl2 as compared to the number of binding sites (six) of the actin polymerized by KCl is taken as evidence of a looser structure of this latter polymer. We have also shown that Tb3+ does not replace 45Ca2+ at the single, "high-affinity" site of G-actin. Removal of this Ca2+, in the presence of Tb3+, destroys the typical G- and F-actin structures.

Actins↗
PubMed1980

The aldolase-substrate intermediates and their interaction with glyceraldehyde-3-phosphate dehydrogenase in a reconstructed glycolytic system.

The relative concentration of the aldolase x fructose-bisphosphate and of the aldolase x dihydroxy-acetone-phosphate complexes is regulated, in the steady state, by the nature of the accompanying glycolytic enzymes. Particularly in the presence of triose phosphate isomerase, the aldolase x dihydroxyactone-phosphate complexes are largely prevalent. This situation is very likely to hold in rabbit muscle in vivo. Aldolase and gyceraldehyde-3-phosphate dehydrogenase slowly form a complex; however, no evidence has been found for the direct transfer of glyceraldehyde 3-phosphate between the two enzymes.

Animals↗
PubMed1979

Fructose-1,6-bisphosphate aldolase from rabbit liver. Reaction mechanism and physiological function.

Liver and muscle aldolase display similar reaction mechanisms. Both the enzymes, by reacting with dihydroxyacetone phosphate, form an acid-labile intermediate which is in rapid equilibrium with an eneamine intermediate. Differences are found in the equilibrium concentration of the acid-labile intermediate, which represents approximately 25% of the total intermediates in the liver (this paper) and 60% in the muscle enzyme [E. Grazi and G. Trombetta, Biochem. J. 175, 361 (1978)] and in the rate of formation of the eneamine intermediate which is much slower in the liver enzyme. Furthermore, with liver aldolase, the rate by which the C-3H bond of dihydroxyacetone phosphate is cleaved is increased by 60 times in the presence of glyceraldehyde 3-phosphate. This, mechanistically, indicates that glyceraldehyde 3-phosphate is bound to the enzyme before the formation of the eneamine from dihydroxyacetone phosphate, and, physiologically, that in liever aldolase the gluconeogenetic activity is favoured over the glycolytic activity.

Animals↗
PubMed1978

A new intermediate of the aldolase reaction, the pyruvaldehyde-aldolase-orthophosphate complex.

Fructose 1,6-bisphosphate aldolase from rabbit muscle forms by reaction with dihydroxyacetone phosphate a pyruvaldehyde-aldolase-orthophosphate complex that is in equilibrium with the eneamine intermediate. The new intermediate accumulates in two phases. The first one is practically complete in 40ms, and the second occurs with an apparent first-order rate constant of 4.6 +/- 0.5s-1. The new intermediate breaks down slowly with the release into the medium of pyruvaldehyde and Pi. The rate of the spontaneous release is higher at acidic than at neutral pH.

Aldehydes↗
PubMed1977

Fructose 1,6-bisphosphate aldolase from rabbit muscle. The isomerization of the enzyme-dihydroxyacetone phosphate complex.

The formation and dissociation of the aldolase-dihydroxyacetone phosphate complex were studied by following changes in A240 [Topper, Mehler & Bloom (1957), Science 126, 1287-1289]. It was shown that the enzyme-substrate complex (ES) slowly isomerizes according to the following reaction: (formula: see text) the two first-order rate constants for the isomerization step being k+2 = 1.3s-1 and k-2 = 0.7s-1 at 20 degrees C and pH 7.5. The dissociation of the ES complex was provoked by the addition of the competitive inhibitor hexitol 1,6-bisphosphate. At 20 degrees C and pH 7.5, k+1 was 4.7 X 10(6)M-1-S-1 and k-1 was 30s-1. Both the ES and the ES* complexes react rapidly with 1.7 mM-glyceraldehyde 3-phosphate, the reaction being practically complete in 40 ms. This shows that the ES* complex is not a dead-end complex. Evidence was also provided that aldolase binds and utilizes only the keto form of dihydroxyacetone phosphate.

Animals↗