The Organic Acids of Rhubarb (rheum Hybridum) * I. on the Malic Acid of Rhubarb, with a Note on the Malic Acid of Tobacco Leaves

The extensive literature on the occurrence of malic acid in plant tissues was reviewed by Franeen and Keyssner (1) in 1923. Although they found that the conclusive tests of isolation and chemical identification had been applied in relatively few cases, there is no question that this acid is widely distributed in nature and is of great importance in plant metabolism. In those instances in which observations of the optical activity have been made, the levorotatory isomer has invariably been found. Previous to 1924 indeed, when Dakin (2) discovered a simple method to resolve commercial synthetic dLmalic acid, cZ-malic acid was an exceedingly rare substance. In two papers published in 1927 and in 1929, Ruhland and Wetzel (3) stated that the rhubarb plant contains not only Z-malic acid, but the dl and d isomers as well, the 1 isomer being found chiefly in the aerial parts, the inactive isomer chiefly in the rhizome and newly developed leaf tissue. They believe that malic acid is involved in the general protein metabolism, being produced along with ammonia by deamination and oxidation of amino acids. If the malic acid of plants originates from the protein amino acids, it should have the same optical configuration as the substances from which it is derived. Pasteur, in 1852 (4), showed