The self-association of calf brain tubulin at pH 7.0 in the presence of magnesium ions has been examined by velocity sedimentation. The schlieren patterns were analyzed by methods described by Gilbert and by Cox. The observed process is best described in terms of a rapidly reversible progressive self-association of the tubulin dimer with identical chain elongation equilibrium constants, k, terminated by a ring-closing step, at degree of polymerization n = 26 +/- 2, with k26 greater than k. The end product of the polymerization reaction has a sedimentation coefficient s20,w0 k2 +/- 2 S. It is hydrodynamically equivalent to a closed ring structure observed in the electron microscope at identical conditions.

A B S T R A C T We investigated the relationship of the kallikrein-kinin system and the renin-angiotensin system in the regulation of blood pressure, salt and water excretion, and renal blood flow. Normotensive and hypertensive black and white men were studied during unrestricted sodium intake as well as on a 10-meq/day sodium intake; potassium intake was held constant throughout the study (80 meq/day).During unrestricted sodium intake, urinary kallikrein activity was greater in white normotensives than white hypertensives or black normotensives. There was no difference (P > 0.05) between white and black hypertensives or between black normotensives and black hypertensives. All groups had greater urinary kallikrein activity on low sodium vs. unrestricted sodium intake, but the increase in black hypertensives was small, and they excreted significantly less kallikrein than the other groups on the low sodium diet. Plasma renin activity showed similar increments after sodium restriction in all groups. Urinary kallikrein activity correlated with renal blood flow in all groups except the black normotensives on low sodium intake. Renal blood flow could be correlated uniformly with log (urinary kallikrein activity/supine plasma renin activity) in all groups on either diet. Urinary sodium and potassium excretion and urine volume were not different among the groups. We concluide: (a) support the concept that the kallikrein-kinin system and the renin-angiotensin system contribute to the regulation of renal blood flow and may account for racial differences in renal vascular resistance. INTRODUCTIONKallikrein is an enzyme that catalyzes the formation of the vasodilator hormones, lysyl-bradykinin (kallidin) and bradykinin, from a plasma alpha-2 globulin substrate, kininogen (1). Urinary kallikrein appears to be derived from the kidney and differs from the enzyme that circulates in plasma (2). It has been proposed that kallikrein exists in the renal cortex and may occupy a location that is juxtaposed to the enzyme renin (3). Although a physiologic function for lysyl-bradykinin, the enzymatic product, has not been established, pharmacologic infusion studies have shown that the peptide is a natriuretic and vasodilatory compound (4).A number of studies in humans and experimental animals have attempted to determine the relationship between urinary kallikrein activity and blood pressure regulation (5-9). Although most investigators feel that kallikreins could influence blood pressure, the meaning of these findings is controversial. Based on provocative preliminary observations (10) and the aforementioned studies, it seemed appropriate to examine the simultaneous influence of race and blood pressture on urinary kallikrein activity. In addition, we have examined the association between kallikrein in uirine and factors that have been related to blood pressure regulation including urinary volume, soditum excretion, plasma renin activity, and renal blood flow. These latter studies were designed to determine a possible physiologic role ...

No abstract

The thermodynamic parameters of the magnesium ion induced self-association of calf brain tubulin in pH 7.0, 0.01 M phosphate buffer containing 10(-4) M GTP, were determined from sedimentation velocity experiments. This reaction proceeds by an isodesmic mechanism terminated by the highly favored formation of a closed ring shaped polymer of degree of association 26 +/- 4. Analysis of the variation of the apparent dimerization constant in the isodesmic mechanism s,ows that this self-association is characterized by positive enthalpy, entropy, heat capacity, and molar volume changes, as well as the binding of one additional magnesium ion, which is probably not involved as a bridge between the protein molecules. The addition of the last monomeric subunit has a free energy which is about three times that of dimer formation. Under the conditions of these experiments, tubulin binds 48 +/- 5 magnesium ions with a free energy of --2.8 kcal/mol.

No abstract