The present invention provides an enzymatic
cycling assay for assessing the amount of
homocysteine and / or cystathionine in a solution such as blood, blood derivatives, or
urine. The
assay comprises the steps of contacting the solution containing
homocysteine and / or cystathionine to form a reaction mixture, with CBS, or a derivative thereof, L-
serine, and CBL, or a derivative thereof, for a time period sufficient to catalyze the cyclical conversion of
homocysteine form to cystathionine and the reconversion of cystathionine to homocysteine with the production of pyruvate and
ammonia; determining the amount of homocysteine and / or
ammonia present in the reaction mixture; and determining the amount of homocysteine and / or cystathionine present in the solution based on the amount of pyruvate and / or
ammonia formed. Expression vectors and isolation procedures for CBS, or derivatives thereof, and CBL, or derivatives thereof, are also provided as well as test kits for carrying out the
assay. In preferred embodiments, the assays can be conducted in 15 minutes or less, with a minimum of
enzyme usage.