Serine palmitoyltransferase (SPT) catalyses the first committed step of de novo sphingolipid biosynthesis. The bacterial SPT homologue from Sphingomonas paucimobilis is a homodimeric enzyme that contains an essential pyridoxal-50 -phosphate (PLP) cofactor bound to each subunit. Inhibitors of SPT are useful blockers of sphingolipid biosynthesis. Here we use UV-vis spectroscopy, enzyme kinetics and mass spectrometry to investigate inhibition of SPT by penicillamine (Pen), a drug with a range of useful medicinal applications.