Abstract
1. A presence of two types of lactonase in the cell fractions of animal tissues has been demonstrated. They were tentatively named Lactonase-I and -II for soluble and microsomal fraction, respectively.
2. Lactonase-I has been purified 21-fold from acetone powder. It re-quired divalent cations such as Mg++ and Mn++ for activation. It has also been shown to have optimum pH 6.8-7.0.
3. Lactonase-I has been shown to have a rather broad substrate specificity and to act on L-gulonolactone, D-glucuronolactone and other lactones, whereas Lactonase-II was specific to glucuronolactone.
4. Lactonase-I has been proved to be an SH-enzyme. Michaelis constant has been determined. Lactonase has been found in the liver of a rat but not in the kidneys, brain, lung, spleen, or heart.
5. The significance of this enzyme in the L-ascorbic acid biosynthesis has been discussed.
The writer's thanks are due to Prof. N. Shimazono and Dr. S. Ishikawa and the other members of this laboratory for their kind guidance and assistance, and to Dr. H. S. Isbell and Dr. T. Nakazima who kindly donated part of the substrates.
