It is an amino acid that helps enhance the immune system.  In response to my vet, Lysine is secure for cats and has no dangerous negative effects.

Hornbeck PV, Kornhauser JM, Tkachev S, Zhang B, Skrzypek E, Murray B, Latham V, Sullivan M: PhosphoSitePlus: a comprehensive resource for investigating the construction and function of experimentally decided publish-translational modifications in man and mouse.

L-Carnitine is a combination of two amino acids