Showing papers in "Journal of Biochemistry in 1968"

Troponin. I. Preparation and physiological function.

Abstract: 1. A method for isolation of troponin from native tropomyosin was described. 2. Troponin in combination with tropomyosin restored the whole activity of native tropomyosin in sensitizing the interaction of myosin and actin to Ca ion. 3. Troponin was found to bind nearly 4 moles of Ca per 10 5 g, of which most were exchangeable. The result of the experiment to determine the binding constant of these Ca binding sites was explained by assuming that half of the binding sites possessed a binding constant of 1.3×10 6 M -1 and the remaining half 5×10 4 M -1 . 4. The amount of exchangeable Ca in the contractile system was mainly accounted for by the Ca-binding capacity of troponin, which was not influenced by other contractile proteins or ATP. 5. Cardiac troponin showed a much higher affinity for Sr ion than skeletal troponin. The ratio of the former affinity to the latter was in good agreement with the ratio of the sensitivity to Sr ion of a reconstituted contractile system containing cardiac troponin to that containing skeletal troponin. Based on these findings and the results described above, it was concluded that the sensitivity of a contractile system to Ca ion is solely dependent upon the affinity for Ca ion of the troponin molecule present. 6. The mechanism of troponin regulation of the interaction of actin and myosin was discussed.

The apparent binding constant of glycoletherdiaminetetraacetic acid for calcium at neutral pH.

Abstract: GEDTA (glycoletherdiaminetetraacetic acid, EGTA), a chelating agent which has a selective affinity for Ca ions compared with Mg ions, has greatly contributed to the clarification of the essential role of Ca ions in the contraction-relaxation cycle (1, 2). Since the concentration of Ca ions involved in physiological process is very low, i.e., not more than 10-5M, indirect measurement by means of a Ca-buffer is more feasible than direct measurement (2, 3). Because of its Ca specificity GEDTA is well suited for this purpose and has, therefore, been widely used in determining the concentration of Ca ions (2, 3). The binding constant of GEDTA used in these experiments was not determined

Relationship between lipase and esterase

Abstract: There are at least two sites on the lipase which are concerned with catalysis: the catalytic site and the hydrophobic recognition site (lipid-binding site). The recognition site may be destroyed by mild proteolytic digestion, but the catalytic site may not be changed by this treatment. Mild treatment with trypsin caused change in the catalytic properties of hepatic triglyceride lipase; the water-insoluble ester-hydrolyzing activity of hepatic triglyceride lipase decreased, whereas the water-soluble ester-hydrolyzing activity did not change. After proteolytic digestion, hepatic triglyceride lipase resembles esterase since it hydrolyzes the water-soluble substrate better than the water-insoluble substrate. Conversely, esterase was converted to lipase by treatment with phospholipid. Cardiolipin in a concentration-dependent fashion enhanced triolein-hydrolysis of human serum carboxylesterase and this effect was associated with a dose-dependent decrease in water-soluble tributyrin hydrolysis. Based on these results, we propose the hypothesis that lipase and esterase have similar catalytic sites and that addition of a hydrophobic recognition site to esterase causes conversion of esterase to lipase (Fig. 9).