Basically for everyone, B2 is crucial for normal activation of B6 pyridoxine (inactive form) to pyridoxal (active form in tissues).

The enzyme that does this task has B2 riboflavin as a cofactor.
Riboflavin also contains ribose, which is not plentiful in the diet.
Ribose is used to make adenosine which is in the mitochrondia where adenosine MP, DP and ATP are made. It has also been found that adenosine outside the cell is the signaling compound for initiating sleep. It is also released in large amounts during acupuncture. One can improve adenosine levels I think with taking some ribose extra... I use d-ribose myself, now and it is quite nice. It is very slow to act (in onset) but does deliver more stamina and better overall sleep. It might be useful in PD, but I have not seen yet studies to that effect. It IS being used in chronic fatigue and fibromyalgia with some success now.

This is why taking a B-complex (which contains riboflavin) is important to getting full benefit of B6 therapy.

Pyridoxal kinsase (the conversion enzyme) may be blocked by some drugs. I recall Theophylline being one. But I don't think research has expanded on many drugs to see their effects on this enzyme. So it can be bypassed by using P5P form of B6, which contains the active form. NOW makes a nice one which is enteric coated to preserve the P5P actions as much as possible. It is not expensive and easily available online.