Scientia Sinica

96 SCIENTIA: SINICA * Vol. V

puted from the data of Tsao, Bailey and Adair", Solid lines indicate the possession of flow birefringence.

It is seen from Figs. 1 and 2 that the polymerizability of pig heart tropomyosin is lower and that of duck gizzard or prawn tropomyosin is higher than that of rabbit tropomyosin. While an ionic strength of 0.01 is sufficient to annihilate the flow birefringence in the case of rabbit tropomyosin, gizzard and prawn tropomyosins require an increase in ionic strength 20-30 times, i.e. 0.2-0.3, to elicit the same effect. It is to be noted that at the relatively high ionic strength of 1-2, the relative viscosity of duck gizzard tropomyosin is still>4, more than twice that for other tropomyosins. This signifies that molecules of the former are either much larger or more asymmetric or both.

Sepia tropomyosin sometimes suffered a loss of polymerizability upon excessive purification while the solubility and other properties remained unchanged. Fig. 2 depicts the viscosity behaviour of such a sample. It serves to bring out the fact that the contribution of electroviscous effect to the measured viscosity is negligible.

2. Osmotic pressure and molecular weight of tropomyosin

Previous investigations have shown that the particle weight of rabbit tropomyosin, owing to its reversible polymerizability and depolymerizability, is a sensitive function of the ionic strength of the dispersing buffer. Bailey, Gutfreund and Ogston'*! estimated the particle weight by ultracentrifugal and osmetic methods and gave a value of about 90,000 for a solution of pH 65 and ionic strength 0.27. Tsao, Bailey and Adair!” discovered that only in strongly depolymerizing media, such as 6.7 M urea and 0.01 N HCl, could the true molecular weight be obtained. In neutral solutions, with the increase in ionic strength, the particle weight decreases drastically at first, and then approaches constant; but the constant value appears to be still slightly higher than the monomeric molecular weight.

We have measured the osmotic pressure of the solutions of gizzard and prawn tropomyosins at pH 2.1. In parallel with the results obtained previously with rabbit tropomyosin?”, these two proteins are stable at this low pH. at 1°C, and are crystallizable and polymerizable after treatment for several weeks. Measurements were also made in neutral salt solutions. Pig heart and sepia tropomyosins are only slightly soluble in 0.01 N HCl, 0.09 M KCl and consequently only neutral salt solutions were employed.

The results are shown in Figs. 3-8. Under each figure are given the buffer composition, the ionic strength I, the linear equation obtained by the method of least squares, the calculated particle weight My and the standard deviation.