Arginine Deiminase from Mycoplasma arthritidis

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 s& +, and D’& values of the enzyme are 5.48 S and 5.87 x 10V7 cm’/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 InM 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% a helix. At pH 2.5, p conformation and disulfide contributions are dominant. Aromatic and cy helix bands are reduced considerably at pH 9.5.