The effect of citrate on the rotation of the molybdate complexes of malate, citramalate and isocitrate.

Extracts of liver and muscle convert citric acid into laevo-rotatory isocitric acid [Martius, 1938]. As the rotation of isocitric acid, like that ofother ac-hydroxy acids, is much increased by (NH4)2MoO4*, the polarimeter can be used to measure low concentrations of isocitric acid in tissue extracts [Martius, 1938; Jacobsohn, 1940]. On applying this method to plant extracts we met with an unexpected complication. Aqueous extracts of rhubarb petioles showed a dextrorotation with molybdate, owing to the presence of ()malic acid. When the extracts were mixed with citrate the dextrorotation increased very considerably, e.g. from + 1.750 to + 3.25°. Investigation of this effect showed that the increase was not caused by an enzymic process; it proved to be due to an effect of citrate on the dextrorotation of the 1(-)malate-molybdate complex. Underthetest conditions [o]c' was + 7160 for 1(-)malic acid in the presence of (NH4)2MoO4 and + 13400 for l()malic acidl in the presence of (NH4)2MoO4 and citrate. Citrate was found also to change the rotation of the molybdate complexes of two substituted malic acids, viz. citramalic and isocitric acids. The effect of citrate on the rotation of the malatemolybdate complex has already been observed by Auerbaoh & Kruger [1923] who realized that citrate interferes with the polarimetric determination of malic acid. Obviously citrate also interferes with the Martius polarimetric isocitric acid estimation. Isocitric acid is frequently accompanied in biological material by citric acid and many of the data calculated from polarimetric readings by previous authors therefore require revision.