sentences:  The mechanism of formation has been studied enzymatically, as well as with tracer experiments, but the exact route of synthesis still remains unclear.Tracer studies using 13C-15N-doubly labeled cadaverine have shown three units of cadaverine are incorporated into sparteine and two of the C-N bonds from two of the cadaverine units remain intact.The observations have also been confirmed using 2H NMR labeling experiments.