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.