 The reason that SDH activity is inhibited by L-cysteine is because an inorganic sulfur is created from L-Cysteine via Cystine Desulfrase and sulfur-containing groups are known to promote inhibition.L-threonine competitively inhibits Serine Dehydratase as well.Moreover, insulin is known to accelerate glycolysis and repress induction of liver serine dehydratase in adult diabetic rats.