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Abstract
The hydrophobicities of ricin D and its constituent polypeptide chains (A- and B-chains) were studied quantitatively. Fluorometric studies using cis-parinaric acid as a probe indicated that the A-chain had 4 times the “surface hydrophobicity” of the B-chain, and the “surface hydrophobicity” of ricin D was lower than that of the B-chain. Density gradient ultracentrifugal analyses using [3H]Triton X-100 indicated that about 300 mol of Triton X-100 were bound per mole of the A-chain in the absence and presence of sodium dodecyl sulfate (SDS), while there is no binding of Triton X-100 to the B-chain in the absence of SDS but 60 mol of Triton X-100 were bound per mole of the B-chain in the presence of 0.02% SDS. No measurable amount of Triton X-100 was bound by ricin D even in the presence of 0.02% SDS. From these findings, it was suggested that the A-chain had larger fractions on its surface which were capable of hydrophobic interaction than the B-chain, and these fractions were masked in the ricin D molecule owing to the iriterfacial contact of the A- and B-chains. The significance of the high hydrophobicity of the A-chain in the function of ricin D is also discussed.