Abstract: An assay method for determining binding of a test compound to pyruvate dehydrogenase kinase (PDHK) at a given site comprising determining (a) the degree of polarisation of the light emitted by a PDHK complex formed by contacting in solution a known concentration of PDHK with a known concentration of a fluorescent compound known to bind to PDHK at that site, and (b) the degree of polarisation of the light emitted by a PDHK complex formed by contacting in solution the test compound, the same concentration of PDHK and the same concentration of the fluorescent compound, a lesser value of the degree of polarisation (b) than that of (a) being taken as indicative of binding of the test compound to that site.