A few years old, but interesting, as there is now a BH4 product on the market (Kuvan)

Information on BH4 (also known as tetrahydrobiopterin): http://en.wikipedia.org/wiki/Tetrahydrobiopterin

Information on Kuvan:
http://www.bmrn.com/products/kuvan.php

Here is the link to the PubMed listing to the abstract below (you can also get this complete article for free).http://www.ncbi.nlm.nih.gov/pubmed/16491266

Asian J Androl. 2006 Mar;8(2):159-67.
Evaluation of tetrahydrobiopterin (BH4) as a potential therapeutic agent to treat erectile dysfunction.
Sommer F1, Klotz T, Steinritz D, Bloch W.
Abstract
AIM: Nitric oxide (NO)-mediated smooth muscle relaxation causes penile erections. The endothelial NO synthase (eNOS) coenzyme tetrahydrobiopterin (BH4) converts eNOS-mediated catalytic activity from oxygen radical to NO production, improving endothelial function and vascular smooth muscle relaxation.
METHODS: Using quantitative immunohistochemistry, 8-isoprostane and nitrotyrosine concentrations were compared in cavernosal tissue from 17 potent and 7 impotent men, and the effect of single oral doses of BH4 on penile rigidity and tumescence was investigated. The pharmacodynamic effect of single oral doses of BH4 on penile rigidity and tumescence was investigated in a randomized, placebo-controlled, double-blind cross-over fashion in 18 patients with erectile dysfunction (ED) while receiving visual sexual stimulation.
RESULTS: 8-Isoprostane content in endothelium and smooth muscle was significantly higher in impotent patient samples; the level of nitrotyrosine was unchanged in ED patients. Relative to placebo, a single dose of 200 mg BH4 led to a mean increase in duration of > 60% penile rigidity (33.5 min [95% confidence interval (CI): 13.1-49.3] at base and 29.4 min [95% CI: 8.9-42.2] at tip). A 500-mg dose increased the relative duration of > 60% penile rigidity by 36.1 min (95% CI: 16.3-51.8) at the base and 33.7 min (95% CI: 11.4-43.9) at the tip. Treatments were well tolerated.
CONCLUSION: BH4 treatment is suggested to switch eNOS catalytic activity from super-oxide to NO formation, leading to a reduced formation of free radical reaction product 8-isoprostane without alteration of nitrotyrosine. The observed results make BH4 a suitable candidate as an ED treatment through reconstitution of altered catalytic activity of the eNOS.

Kuvan is for treating PKU – the disease that makes NutraSweet (aspartame) dangerous. Do people with ED also have a problem with aspartame? Maybe there is a common link.