Hydrolysis of human adult hemoglobin (Hb A) by cathepsin, pepsin, papain, and trypsin, and the effects thereon of haptoglobin (Hp) were examined.
Free Hb A was easily digested by cathepsin and pepsin at acidic pH, while Hb A bound by Hp resists digestion by these proteases. Protective effect of Hp on the acid denaturation of Hb A may have some connection with the difference in the susceptibility to cathepsin and pepsin between free Hb A and Hb A bound by Hp. At neutral pH, Hp also diminishes the susceptibility of Hb A to papain and trypsin. Hb A-Hp complex has an inhibitory effect on the digestion of casein by trypsin, and this inhibition seems to be a non-competitive type.
The resistance of Hb A bound by Hp to hydrolysis by those proteases may be one possible explanation for the stronger antigenicity of Hb A bound by Hp rather than free Hb A.
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