Cellular effects of proteinase activity in commercial xanthine oxidase

Xanthine oxidase activates pro-matrix metalloproteinase-2 in cultured rat vascular smooth muscle cells through non-free radical mechanisms

Reactive oxygen species (ROS) have been implicated in the regulation of matrix metalloproteinases (MMPs). The xanthine/xanthine oxidase (X/XO) reaction has been widely used as a source of exogenous ROS in studying MMPs, but commercial XO has also been known to be contaminated by proteolytic activity, and MMPs are protease sensitive substrate. We have investigated the activation of proMMP-2 by X/XO in cultured vascular smooth muscle cells (SMCs). SMCs were incubated with X/XO (unpurified or purified) or XO alone for 24h. X/XO activated proMMP-2 in a dose-dependent manner. A similar profile was observed using XO. Purified XO produced lower amounts of active MMP-2 compared to unpurified XO. EPR study showed that X/XO, not XO itself, produced superoxide anion, which was completely scavenged by SOD. However, X/XO-induced proMMP-2 activation could not be inhibited by combination of SOD and catalase. Incubation with XO either in cell-free conditioned media or in cells resulted in similar amounts of active MMP-2, suggesting that membrane-type-MMPs were not involved in proMMP-2 activation. This was further confirmed by the lack of inhibitory effect of hydroxamate MMP inhibitor, BB1101. Aprotinin blocked unpurified XO-induced proMMP-2 activation in a dose-dependent manner, demonstrating the proteolytic activity contained in XO is essential. We conclude that proteolytic activity contained in XO, rather the ROS derived from X/XO, is responsible for proMMP-2 activation in cultured SMCs. The results also suggest that caution needs to be taken when interpreting the reported results on activation of MMPs where X/XO had been used as an "authentic" source of superoxide anion.

Acute effects of a superoxide radical-generating system on DNA double-strand stability in Chinese hamster ovary cells. Determination by a modified fluorometric procedure

The rate of oxyradical generation by a xanthine oxidase-xanthine system to acutely cause DNA strand breakage in Chinese hamster ovary cells was studied in a phosphate-buffered saline system. DNA strand breakage, measured by a fluorometric procedure, was found to increase curvilinearly as a function of oxyradical generation. Results of studying the ability of 5 mM mannitol, 10 mM dimethylthiourea, 300 micrograms superoxide dismutase/ml, or 1 mg catalase/ml to interfere with DNA damage at a high rate of oxyradical production best supported a hydrogen peroxide-promoted mechanism for DNA breakage.

A new technique for highly sensitive detection of superoxide dismutase activity by chemiluminescence

A stable enzymatic free radical generation system has been developed which allows a precise production of 02-. and its detection by chemiluminescence between 2 pmol and 8 nmol. This test has been used for assaying superoxide dismutase (SOD) by inhibition of the chemiluminescence (CL) signal. No inhibition was observed with catalase, which excludes the participation of H2O2 in lucigenin CL. N,N-Diethyldithiocarbamate gives 100% inhibition of SOD activity either from a purified enzymatic preparation or from biological samples, which confirms the specificity of the CL assay. SOD assay can be performed either on a purified enzymatic preparation or on biological materials such as cultured cells.

Contamination of commercial preparations of xanthine oxidase by a Ca2+-dependent phospholipase A2

Using [1-14C]oleate-labelled autoclaved Escherichia coli as substrate, we demonstrate that many, but not all, commercial preparations of xanthine oxidase contain phospholipase A2 activity as a contaminant. Phospholipase A2 activity (64.3-545.6 nmol phospholipid hydrolyzed per min per mg protein) was optimal in the neutral to alkaline pH range, was Ca2+-dependent, and was unaffected by the addition of xanthine. Phospholipase A2 activity was totally inhibited by 1.0 mM EDTA while radical production by xanthine plus xanthine oxidase was unaffected by EDTA. Even chromatographically purified xanthine oxidase (Sigma Grade III) contained substantial phospholipase A2 activity (64.3 nmol/min per mg). Since the preparation of xanthine oxidase employs proteolytic digestion of milk or buttermilk by pancreatin, an extract of pancreas which is an organ rich in phospholipase A2 activity, we speculate that the contaminant phospholipase A2 is introduced by this treatment. Because xanthine oxidase is used extensively to study free radical-induced cell injury and membrane phospholipid alterations, the presence of a potent extracellular phospholipase A2 may have influenced previously published reports and such studies in the future should be interpreted with care.