Purification and partial characterization of three isoforms of serine hydroxymethyltransferase from Crithidia fasciculata.

Three molecular forms of serine hydroxymethyltransferase (SHMT) have been detected in choanomastigotes of Crithidia fasciculata by DEAE-cellulose chromatography. The three isoforms (named SHMT I, II, and III) presented small differences in charge and molecular weight. Digitonin treatment of intact cells suggested that SHMT III is cytosolic, whereas the other two isoforms are particle bound, one being mitochondrial (SHMT I) and the other one very likely glycosomal (SHMT II). The three SHMT isoforms were purified to homogeneity, and their physicochemical and kinetic properties studied. Determination of their native and subunit molecular masses revealed that all of them have a tetrameric structure. The three isoforms were shown to be PLP-dependent enzymes after L-cysteine and hydroxylamine hydrochloride treatments. They showed similar pH optima, bimodal kinetics for L-serine and Michaelis-Menten kinetics for THF.