Arginine deiminase from Mycoplasma arthritidis. Properties of the enzyme from log phase cultures

Abstract

Hydrodynamic, chemical, and optical properties of arginine deiminase (EC 3.5.3.6) from Mycoplasma arthritidis are reported for the enzyme isolated from log phase cells. The S020,w and D020,w values of the enzyme are 5.48 S and 5.87 X 10(-7) cm3/s, respectively; the molecular weight is 87,300. Determination of the amino acid composition shows that about 45% of the residues are nonpolar. Another unique feature of the composition is the presence of 36 half-cystine residues. The state of oxidation of the half-cystines appears to be well established as 16 disulfide and 4 sulfhydryl groups. The reaction of 1 sulfhydryl group with 0.3 mM 5,5'-dithiobis(2-nitrobenzoic acid) has a half-life of about 50 min at pH 8. The modified enzyme retains full activity. Two -SH groups are accessible to this reagent in 2 M guanidine hydrochloride, whereas all 4 -SH groups react immediately in 4 M guanidine hydrochloride. Reduction of disulfide bonds with dithiothreitol occurs only to a limited extent in 8 M urea, but is complete in 4 M guanidine hydrochloride. The enzyme loses activity immediately at pH 2.5, but retains full activity upon standing 8 h at pH 9.5 in several buffers. The large number of cystine residues leads to a complex near ultraviolet circular dichroism spectrum with cystine contributions apparently superimposed on contributions from aromatic residues. The far ultraviolet spectrum suggests that the molecule contains about 18% alpha helix. At pH 2.5, beta conformation and disulfide contributions are dominant. Aromatic and alpha bands are reduced considerably at pH 9.5.