Understanding L-Lysine: What's the 'L' Mean?

Question
what does the "L" in front of a vitamin name mean?

Thanks

Answer
Bryan,

The letter L in front of a chemical substance's name refers to its belonging to so called optical isomers. It means that molecules' geometry can take one of the two mirror forms: L or D. This kind of isomers are called enantiomers. I know of no enantiomers of vitamins. You might mean L-Lysine, which is an amino acid and the only biologically active form. Its mirror twin, D-Lysine, is the non-physiological isomer of L-lysine.

= TZ