Prune juice helps reduce, but does not cure, my problems with constipation.

What causes its effectiveness?

Van Gorsel et al.[1] analyse the composition of various fruit juices. Prune juice comes out relatively low in citric, ascorbic, malic and tartaric acids, but (along with kiwi fruit) high in quinic acid. Also (along with grape) it has relatively high levels of tyrosine.

Given that prune juice has a bigger effect on me than other juices, I'm inclined to focus on the differences between prune juice and other juices.

Tyrosine, although a precursor to levodopa and hence to dopamine, is unlikely to be important here, because for most people the critical path in dopamine synthesis is not tyrosine per se, but the catalyst tyrosine hydroxylase. [2]

So, let's focus on quinic acid.

Can it get through the blood brain barrier?
Probably. Wikipedia [3] states: "In rats, theogallin, or its metabolite quinic acid, can move through the blood–brain barrier and can have cognition enhancing activities".

Can it be converted to dopamine in the brain?
I don't know. But, exasperatedly, there's a paper that I can't access called:
Chem Ber. 1968;101(9):3313-25.
[Conversion of quinic acid into dopamine and noradrenaline].
[Article in German]
Fischer M, Friedrichsen W, Grewe R, Haendler H.
Can anyone access this paper?

Are there any references to its usefulness in PD?
I've not been able to find any mention of its use specifically in PD. However, a recent paper by Lee et al. [4] states "Quinic acid derivatives have various
beneficial effects including antioxidant, anti-inflammatory, anti-HIV, anti-hepatitis B virus, hypoglycaemic, and hepatoprotective activities and inhibition of mutagenesis and carcinogenesis." [4]

Reference

[1] J. Agric. Food Chem, 1992, 40, 784-789
"Compositional Characterization of Prune Juice"
Hendrik van Gorsel, Chingying Li, Eduardo L. Kerbe1,t Mirjam Smits,t and Adel A. Kader'

[2] Wikipedia, "Tyrosine hydroxylase"
http://en.wikipedia.org/wiki/Tyrosine_hydroxylase

[3] Wikipedia, "Theogallin"
http://en.wikipedia.org/wiki/Theogallin

[4] "Quinic acid derivatives from Pimpinella brachycarpa exert
anti-neuroinflammatory activity in lipopolysaccharide-induced
microglia"
Seung Young Lee a, Eunjung Moon b, Sun Yeou Kim b,c, Kang Ro Lee a,
Bioorganic & Medicinal Chemistry Letters 23 (2013) 2140–2144
naturalproduct.skku.edu/erp/erpmenus/professor.../bmcl_lsy.pdf

John