Scientia Sinica

100 SCIENTIA. SINICA Vol. V The results are summarized in Table I.

Table 1. Molecular Weights of Tropomyosins from Different Sources.

Tropomyosin pH 2 (1=0.1) pH 7 (I1=1.6) Note Rabbit striated muscle 52,700-1,800 ~65,000* S0)T PHC Duck gizzard smooth muscle 147,000+4,000% | 153,000+45,500 *1=0.2 Prawn striated muscle 74,000+5,000 80,000+3,000 ‘ Sepia mantle smooth muscle 68,000+2,000 Pig cardiac muscle 89,000-+4,000

It is seen from Table I that the particle weight of duck gizzard tropomyosin is about three times that of rabbit tropomyosin. The values obtained in a dilute acid at pH 2.1 and in a phosphate buffer (pH 7.0, I=0.6) agree well with each other. Considering also (Fig. 1) that at I=0.6 the curve 7/4o—-1 already flattens and that the value of 7/7 is very high at this and . even higher ionic strengths, we presume that salt ions alone at a sufficient concentration are capable of bringing about complete depolymerization of this protein. The experimental values, 147,000, or 153,000 (average 150,000), can therefore be regarded as the monomeric molecular weight of duck gizzard tropomyosin. Likewise, the molecular weight of prawn tropomyosin lies near 77,000, being the average of 74,000 and 80,000 obtained under two sets of depolymerizing conditions.

Osmotic pressure measurements for pig heart and sepia mantle tropomyosins have only been carried out in a phosphate buffer (pH 7.0, I=1.6). Judging from the viscosity results depicted in Fig. 2, where it is shown that the depolymerization of these two proteins is already complete long before an ionic strength of 1.6 is reached, and from the fact that a sample of sepia tropomyosin was used which had lost the ability to polymerize, it is permissible to assume that the experimental values, 89,000 and 68,000, obtained in this buffer represent the monomeric molecular weights of pig heart and sepia mantle tropomyosins respectively.

It is evident that the molecular size of tropomyosin varies greatly from species to species, at least when the positions on the phylogentic scale are far apart, or when the physiological functions considerably differ.

Since in the phosphate buffers employed salt ions were present in abundance (I=0.6-1.6) and proteins were only about 2 units from their isoelectric points, it is safe to assume that the Donnan effect can be neglected. An estimate was made of the molecular asymmetry of tropomyosin on the basis of Zimm and Onsager’s theory (see Edsall!) from the the slope m and the