The effects of ionic strength on the structure of yeast sulfite reductase (EC 188.8.131.52) were investigated. Gel filtration and sedimentation analysis revealed that this enzyme dissociated at ionic strength below 0.1 into two components having molecular weights of 300000 and sedimentation coefficients of 7-9 S. The secondary structure and the surroundings of the prosthetic groups were not appreciably affected by ionic strength. This enzyme showed CD spectra in the near-ultraviolet region considered to be due to tyrosine residues. On lowering the ionic strength, they decreased, together with the disappearance of the difference spectra in the same region, indicating that some tyrosine residues became exposed to the environment at low ionic strength. SH groups were also exposed at low ionic strength and reacted with DTNB and PCMB more easily to cause the loss of NADPH-sulfite reductase activity. These essential SH groups as well as some tyrosine residues may exist in the region of contact between the two components constituting the enzyme molecule.
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