PPIG and chronic obstructive pulmonary disease: Smoking is a key factor for COPD pathogenesis, it can bring a lot of oxygen free radicals into the lungs, triggering oxidative stress and directly damaging lung tissue; while in the antioxidant system, antioxidant enzymes (including: glutathione S-transferase (GST), epoxide hydrolase exon (EPHX), superoxide dismutase (SOD), catalase (CAT) and cytochrome P450 system (CYP), etc.)provide endogenous biological defense against oxidative stress [4].