Acetolactate Synthase Isozyme III from Escherichia coli

Acetolactate synthase (ALS, EC 4.1.3.18), also known as acetohydroxyacid synthase, is an enzyme which catalyzes the condensation of pyruvate
either with another pyruvate molecule or with a-ketobutyrate to form
a-acetolactate or a-aceto-a-hydroxybutyrate, respectively.l-3 ALSIII is one
of the two isozymes functioning in Escherichia coli K12. '-a It is encoded
by the ilvlH operon, which is regulated by leucine alone, 2-s and is closely
linked to the IeuABCD operon. 4 Although the gene is transcribed, this
isozyme is cryptic in Salmonella typhimurium. 6 The lability of ALSIII like
that of the other isozymes, has hampered its isolation, despite the availability of bacterial clones capable of its overproduction. 3,7,8 Hence, all characterizations of ALSIII have been based on crude or partially purified preparations from mutants which synthesize this single isozyme only. 2'9 This article describes the procedure for purification of ALSIII and the properties
of the isolated enzyme.