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

Three molecular forms of serine hydroxymethyltransferase (SHMT) have been d etected in choanomastigotes of Crithidia fasciculata by DEAE-cellulose chro matography. The three isoforms (named SHMT I, II, and III) presented small differences in charge and molecular weight. Digitonin treatment of intact c ells suggested that SHMT III is cytosolic, whereas the other two isoforms a re particle bound, one being mitochondrial (SHMT I) and the other one very likely glycosomal (SHMT II). The three SHMT isoforms were purified to homog eneity, and their physicochemical and kinetic properties studied. Determina tion of their native and subunit molecular masses revealed that all of them have a tetrameric structure. The three isoforms were shown to be PLP-depen dent enzymes after L-cysteine and hydroxylamine hydrochloride treatments. T hey showed similar pH optima, bimodal kinetics for L-serine and Michaelis-M enten kinetics for THF. (C) 1999 Elsevier Science B.V. All rights reserved.