The behavior of holo- and apo-forms of bovine superoxide dismutase at low pH

Abstract

1. 1. Holo-superoxide dismutase from bovine erythrocytes has been shown to undergo a reversible structural modification in the pH 3–5 range. 2. 2. The spectral alterations observed on changing from neutrality to pH 3 were: a slight attenuation of the 680 nm absorbance; the loss of the 450 nm shoulder, apparent in the optical spectrum of the native protein; and a new band appeared at 330 nm. The circular dichroism at 600 nm was essentially lost while a weak negative band appeared at approx. 380 nm and a positive band at 310 nm. 3. 3. The EPR spectrum was also modified on changing from the native to the low pH form: A | increased from ≈ 130 to ≈ 150 G, g | remained unchanged at ≈ 2.27, and g m decreased from ≈ 2.09 to ≈ 2.08. The apparent linewidth remained essentially constant. 4. 4. High resolution (220 MHz) PMR spectra of holo- and apoproteins revealed that the metals influence the three-dimensional structure of the protein. 5. 5. PMR studies indicated that at pH 3 the apoprotein existed almost entirely in a random coil form and that it assumed a compact well-ordered structure on returning to neutral pH. The holoprotein maintained a compact, apparently dimeric, structure even at pH 3.