We originally identified senescence marker protein 30 (SMP30) as a distinctive protein whose expression decreases in an androgen-independent manner with aging. Here, we report its sequence homology found in two kinds of bacterial gluconolactonases (GNLs) by using the blast search. Then, through a biochemical study, we identify SMP30 as the lactone-hydrolyzing enzyme GNL of animal species. SMP30 purified from the rat liver had lactonase activity toward various aldonolactones, such as d - and l -glucono-δ-lactone, d - and l -gulono-γ-lactone, and d - and l -galactono-γ-lactone, with a requirement for Zn ²⁺ or Mn ²⁺ as a cofactor. Furthermore, in SMP30 knockout mice, no GNL activity was detectable in the liver. Thus, we conclude that SMP30 is a unique GNL in the liver. The lactonase reaction with l -gulono-γ-lactone is the penultimate step in l -ascorbic acid (AA) biosynthesis, and the essential role of SMP30 in this synthetic process was verified here by a nutritional study using SMP30 knockout mice. These knockout mice ( n = 6), fed a vitamin C-deficient diet, did not thrive; i.e., they displayed symptoms of scurvy such as bone fracture and rachitic rosary and then died by 135 days after the start of receiving the deficient diet. The AA levels in their livers and kidneys at the time of death were <1.6% of those in WT control mice. In addition, by using the SMP30 knockout mouse, we demonstrate that the alternative pathway of AA synthesis involving d -glucurono-γ-lactone operates in vivo , although its flux is fairly small.

Rationale: Senescence marker protein-30 (SMP30) is a multifunctional protein providing protection to cellular functions from age-associated deterioration. We previously reported that SMP30 knockout (SMP30Y/−) mice are capable of being novel models for senile lung with age-related airspace enlargement and enhanced susceptibility to harmful stimuli. Objectives: Aging and smoking are considered as major contributing factors for the development of pulmonary emphysema. We evaluated whether SMP30Y/− mice are susceptible to oxidative stress associated with aging and smoking. Methods: Age-related changes of protein carbonyls in lung tissues from the wild-type (SMP30Y/+) and SMP30Y/− mice were evaluated. Both strains were exposed to cigarette smoke for 8 wk. Histopathologic and morphologic evaluations of the lungs, protein carbonyls and malondialdehyde in the lung tissues, total glutathione content in the bronchoalveolar lavage fluid, and degree of apoptosis of lung cells were determined. Measurements and Main Results: In the lungs of SMP30Y/− mice, protein carbonyls tended to increase with aging and were significantly higher than the age-matched SMP30Y/+ mice. Cigarette smoke exposure generated marked airspace enlargement (23.3% increase of the mean linear intercepts) with significant parenchymal destruction in the SMP30Y/− mice but not in the SMP30Y/+ mice (5.4%). The protein carbonyls, malondialdehyde, total glutathione, and apoptosis of lung cells were significantly increased after 8-wk exposure to cigarette smoke in the SMP30Y/− mice. Conclusions: Our results suggest that SMP30 protects mice lungs from oxidative stress associated with aging and smoking. The SMP30Y/− mice could be useful animal models for investigating age-related lung diseases, including cigarette smoke–induced pulmonary emphysema.