Effect of solution variables on the binding of vinblastine to tubulin.

Vinblastine binding to tublin was measured in different buffers using tubulin prepared by two different methods and three different binding assay methods. In 100 mM 1,4-piperazinediethanesulfonic acid (Pipes) buffer containing 1 mM MgSO4 and 1 mM ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), the data appeared to be consistent with one site with a Ka value of 3.4 X 10(6) M-1 and another site with a Ka value of 2.8 X 10(5) M-1. However, in buffers of lower ionic strengths and without Mg2+ the Ka values were lower. The lowest value (2 X 10(4) M-1) was obtained in 10 mM phosphate buffer, in which only one site was evident under the conditions used. Neither the binding assay used nor the method for tubulin preparation affected the Ka value. Using HPLC, aggregation induced by vinblastine was evident in buffers which gave the largest Ka values. Tubulin aggregation in the presence of vinblastine was also confirmed by analytical ultracentrifugation. The results support the proposal of Na and Timasheff [Biochemistry 25, 6214 (1986)] that the apparent Ka value is influenced by the degree of aggregation induced by vinblastine and that the intrinsic binding constant to the dimer is represented by the lowest value, about 2 X 10(4) M-1.

Protease nexin. Properties and a modified purification procedure.

The present paper describes chemical and functional properties of protease nexin, a serine protease inhibitor released from cultured human fibroblasts. It is shown that protease nexin is actually synthesized by fibroblasts and represents about 1% of their secreted protein. Analysis of the amino acid composition of purified protease nexin indicates that it is evolutionarily related to antithrombin III and heparin cofactor II. Protease nexin contains approximately 6% carbohydrate, with 2.3% amino sugar, 1.1% neutral sugar, and 3.0% sialic acid. The Mr calculated from equilibrium sedimentation analysis is 43,000. Protease nexin is a broad specificity inhibitor of trypsin-like serine proteases. It reacts rapidly with trypsin (kassoc = 4.2 +/- 0.4 X 10(6) M-1 s-1), thrombin (kassoc = 6.0 +/- 1.3 X 10(5) M-1 s-1), urokinase (kassoc = 1.5 +/- 0.1 X 10(5) M-1 s-1), and plasmin (kassoc = 1.3 +/- 0.1 X 10(5) M-1 s-1), and slowly inhibits Factor Xa and the gamma subunit of nerve growth factor but does not inhibit chymotrypsin-like proteases or leukocyte elastase. In the presence of heparin, protease nexin inhibits thrombin at a nearly diffusion-controlled rate. Two heparin affinity classes of protease nexin can be detected. The present characterization pertains to the fraction of protease nexin having the higher affinity for heparin. The low affinity material, which is the minor fraction, is lost during purification.

Ribosomes and ribonucleic acids of Coxiella burneti.

This report describes the direct isolation and characterization of rickettsial ribosomes. Ribosomes from the rickettsia Coxiella burneti were isolated and partially characterized. The ribosomes had a sedimentation constant of about 70S and could be dissociated into 50 and 30S subunits. Electron microscopy revealed ribosomal particles with dimensions similar to those reported for other procaryotic organisms. Ribonucleic acid (RNA) species (23 and 16S) were isolated from the ribosomal particles. The nucleotide compositions of the ribosomal RNAs were found to be similar to those reported for bacterial ribosomal RNA. In addition to the high-molecular-weight ribosomal RNA, 5S RNA was also extracted from the organism.