Free acid.
I see no reason to add leucine. The reason leucine works is because your body metabolizes it to HMB.
Here's the skinny on how it works.
Our bodies are in a constant state of building and tearing down. Humans are still not fully evolved for a life of abundance. We are set up to be in a constant state of preparation for starvation and thus we resist building any more muscle than we need to, because muscle is "expensive" to maintain, and our bodies are still paranoid that there is no.guarantee that the food will keep coming, so don't build anything you don't absolutely need and go ahead and tear down whatever you can so you aren't stuck maintaining something you can't afford. This is amplified further once we are past reproductive age. We are designed to deteriorate once we've raised our kids because past that point all we do is compete with them for resources.
Leucine is an essential amino acid (we cannot make it ourselves and must get it from our food). It is metabolized in part to HMB. It is relatively uncommon in most (not all) non-animal foods. Thus, the level of HMB in your blood serves as an indicator to your body of the leucine available to you and in turn the quality of protein available to you.
By increasing serum HMB concentrations, your body is being told "high quality protein is consistently available. You can chill out at breaking down protein."
It was discovered in the late 80s that adding leucine to animal feed made the animals grow faster (there's a hilarious movie about it with Matt Damon called "The Informant"). For some bewildering reason it took a while for science to wonder if it did the same thing to humans. It was first thought to be all BCAAs but later found to be primarily due to the leucine.
As it turns out, HMB directly fed to you works better than leucine supplementation.