Mannitol metabolism in the red alga Caloglossa continua

Abstract

Mannitol is one ofthe most widely occurring sugar alcohol compounds. The substances are found in bacteria, fungi, algae, and plants. In these organisms, the compound acts as a compatible solute and has multiple functions, such as osmoregulation, storage ofcarbon and reducing power, and scavenging ofactive oxygen species. Because of the diverse functions of mannitol, the attempts to introduce the mannitol biosynthesis pathway into the useful plants to generate salt tolerant transgenic plants have been conducted. However, transgenic plants have not yet improved significantly in their salt tolerance. Though the reason is not specified yet, introducing the mannotol biosynthetic pathway possessing superior enzymatic characteristic would be one ofthe solution. In this report, the author show mannitol-l-phosphate dehydrogenase and a mannitol-l-phosphatase, from the marine red alga Caloglossa continua, which grows in estuarine areas where tide levels fluctuate frequently. 1. Mannitol-lPhosphate Dehydrogenase (MIPDH)."MIPDHcatalyzes the reduction offructose-6-phosphate (F6P) to mannitoll- phosphate (M I P). The enzyme was purified to homogeneity from the red alga, C. continua. The enzyme was a monomer with an apparent molecular mass of 53 kD. The substrate specificity was very high toward F6P and MIP for respective reductive and oxidative reactions. Interestingly,the Vmta andK; oftheMIP reducing activities were greatly influenced by several parameters such as substrate concentration and ionic strength of the environment. Because this enzyme regulates the activity in the enzyme level, the introduction of this enzyme has an advantage in keeping the carbon and energy metabolism in the cell. 2. Mannitol-I-Phosphatase (MIPase). MIPase catalyzes the final step of the mannitol biosynthetic pathway and was also purified to homogeneity from the red alga, C. continua. The enzyme was shown to be a monomer by gel-filtration and SDS-PAGE with molecular mass of ca. 28 kD. The enzyme showed high substrate specificity for mannitol-l-phosphate with the low Km (MIP) value ofO.4ImM, which is the most advantageous not to disturb the metabolism of other phosphate compounds.