血合筋肉の化学的研究-XII : マグロ・ミオグロビンの酸素解離曲線

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The oxygen dissociation curve of myoglobin (Mb) of tuna, Thunnus orientalis, was measured on crystalline specimen of the pigment essentially by the method of RIGGS7). According to the results obtained, the respiratory property of tuna Mb was as a whole very similar to that of mammalian Mbs: Its oxygen affinity was hardly affected by pH, and the concentrations of salt and the pigment, but appreciably by temperature (Figs. 1 and 2). The overall heat of oxygenation calculated from these data was 14.6 kcal per mole of Mb. The oxygen pressure at half saturation for tuna Mb was around 0.6mm. Hg at neutral pH and 15°C., whose value was also in a good agreement with those for mammalian. The Mb is, howeves, different from mammalian ones in the following point: Although it is reported10) that there is a proportional relationship for Hb and Mb between the number of free sulfhydryl group in their molecule and the degree of the BOHR effect, it is not the case with tuna Mb, since it possesses one to two free sulfhydryl groups per molecule6, 18) and neverthless its oxygen affinity is scarcely affected by pH. The relative affinity of tuna Mb for oxygen and carbon monoxide was, however, some what characteristic; the equilibrium constant (K), as defined by the equation, K= ?? MbCO ?? •po2/ ?? MbO2 ?? •poo, was fairly larger than those for mammalian Mbs (Table 1). A linear relationship between logarithm of the equilibrium constant and the span (the difference in wavelength between the α bands of oxy and carbonyl-derivative of Mb or Hb) which has been reported by THEORELL15) and others, was found to be inapplicable to this Mb (Fig. 3).

血合筋肉の化学的研究-XII : マグロ・ミオグロビンの酸素解離曲線

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