Labile sulfur in human Superoxide dismutase.

Area of Science:

• Biochemistry
• Enzymology

Background:

• Superoxide dismutase (SOD) is a crucial enzyme protecting against oxidative stress.
• Human red blood cell SOD contains copper and zinc, and exhibits an intense absorption band at 320 nm.

Purpose of the Study:

• To investigate the nature of the 320 nm absorption band in human superoxide dismutase.
• To determine the chemical basis of this spectral feature and its relationship to enzyme activity and metal cofactors.

Main Methods:

• Spectroscopic analysis (UV-Vis absorption) to study the 320 nm band.
• Chemical treatments including metal removal (cyanide), reduction (dithiothreitol, sulfite, borohydride), denaturation (guanidine HCl, SDS), and pH variation.
• Quantitative sulfur determination.
• Enzyme activity assays and electron paramagnetic resonance (EPR) spectroscopy.

Main Results:

• The 320 nm band is present in the apo protein (metal-free) but lost upon copper removal with cyanide.
• The band is abolished by reducing agents, denaturants, and extreme pH values.
• Four sulfur atoms per protein molecule are associated with the 320 nm chromophore.
• A persulfide group (R-S-SH) is proposed as the most likely structure for the chromophore.
• Bovine and equine SOD lack spectral or chemical evidence for this labile sulfur group.

Conclusions:

• The 320 nm absorption band in human SOD is attributed to a labile sulfur group, likely a persulfide.
• This sulfur moiety is distinct from the copper prosthetic group and is not required for superoxide dismutase activity.
• The labile sulfur group is specific to human SOD and not found in bovine or equine counterparts.