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-delta-lactone, d- and l-gulono-gamma-
lactone, and d- and l-galactono-gamma-
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-gamma-
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-gamma-
lactone operates in vivo, although its flux is fairly small.