High Tower Pharmacology: September 2012

Introduction
Amentoflavone is a polyphenolic compound extracted from many different plants including Ginkgo biloba and St John's Wort. It has received most of its attention due to its anti-cancer and anti-microbial properties. This article will focus on its lesser known mechanisms which are more relevant in the context of athletic performance.

Background
I first became aware of amentoflavone in 2005 when a research article was published demonstrating its ability to negatively modulate the benzodiazepine GABA(A) receptor site (1). This modality is a potential way to enhance learning and memory since it would disinhibit excitatory neurotransmission. It could also increase the production of testosterone by inducing the release of GnRH at the hypothalamus. Unfortunately, as its structure should remind us, amentoflavone has almost no capacity to cross the blood brain barrier as was demonstrated in the 2008 study by Colovic et al (2). This is not necessarily terrible news, as GABA(A) antagonism brings along with it a host of potentially dreadful side effects from anxiety to the potential for neurotoxicity. Luckily, its peripheral properties are significant enough to warrant further investigation.

Phosphodiesterase Inhibition
Phosphodiesterase (PDE) is an intracellular enzyme which degrades the second messengers cAMP or cGMP. In human adipose tissue, beta-2 agonism results in an increase in cAMP which activates lipases that cause cellular fat breakdown ("lipolysis"). By inhibiting the particular phosphodiesterase isoenzyme (PDE3) found in adipose tissue, a compound could theoretically synergize with the adrenergic signaling cascade and induce significant fat loss. Indeed, amentoflavone has demonstrated this capacity in a 1998 Italian study examining the effect of Ginkgo biloba on rat adipose tissue (3).

This work compares the inhibition of cAMP-phosphodiesterase in rat adipose tissue by a mixture of Ginkgo biloba biflavones with the effect of individual dimeric flavonoids. The degree of enzyme inhibition by G. biloba biflavones was amentoflavone > bilobetin > sequoiaflavone > ginkgetin = isoginkgetin.

A 2006 Planta Medica article also identified amentoflavone as a weak inhibitor of PDE5, although having much greater inhibitory capacity for other isoforms (4, 5). The former PDE is responsible for the metabolism of cGMP, whereas the latter isoforms deal mainly with cAMP. Inhibiting cGMP disposal allows for vascular dilation (i.e. Viagra) via smooth muscle relaxation. Inhibiting cAMP metabolism potentiates various transduction cascades including lipolysis in adipose tissue, as discussed above, and enhancing cardiac contractility and speed (6).

Muscular Strength
Amentoflavone was recently demonstrated to possess acetylcholinestase inhibiting properties in a 2011 study (7). By inhibiting AchE, more acetylcholine ligand would be available at the neuromuscular junction, disinhibiting Ach metabolism from being a rate limiting step for muscular contraction. Unfortunately, AchE inhibition alone has not demonstrated an ability to enhance muscular strength in healthy individuals (8). Fortunately, however, amentoflavone possesses another modality that may synergize well with AchE inhibition: enhancing calcium release from the sarcoplasmic reticulum.

The Ca2+ -releasing activity of amentoflavone was approximately 20 times more potent than that of caffeine...These results suggest that amentoflavone, which does not contain a nitrogen atom, probably binds to the caffeine-binding site in Ca2+ channels and thus potentiates Ca2+ -induced Ca2+ release from the heavy fraction of fragmented sarcoplasmic reticulum.

This is a novel mechanism for enhancing muscular contraction and one of the ways in which caffeine increases strength, albeit weakly (9). Since amentoflavone is approximately 20 times more potent then caffeine, it is also possible that it could exert greater efficacy in this area.

Other Mechanisms
Amentoflavone, in addition to its exceptionally weak ability to inhibit fatty acid synthase (10) and ability to potentiate cAMP in adipose tissue, also possesses another novel metabolic mechanism: Protein tyrosine phosphatase 1B (PTP1B) inhibition (11).

Regulation of protein phosphatases in disease and behaviour; EMBO reports (2003) 4, 1027 - 1031 doi:10.1038/sj.embor.7400009

PTP1B is an negative regulator of the growth promoting cascade induced by tyrosine kinase receptors. By inhibiting PTP1B, amentoflavone disregulates the downstream pathways activated by various ligands, including those induced by insulin. This could have an exceptionally beneficial effect in relation to insulin insensitivity, or just as a means to potentiate insulin itself. Unfortunately, it could also have pro-oncogenic outcomes in those with cancer. Needless to say, any growth promoting compound (estrogen, GH, IGF-1, DHT, et cetra) has the capacity to stimulate oncogenesis, and so this mechanism should not be hysteria-provoking - especially in light of amentoflavones other anti-cancer modalities (anti-mutagenesis, anti-angiogenesis).

Summary

PDE inhibition (multiple isoforms)

Weakly vasodilatory
Capacity to potentiate adrenergic signaling in adipose tissue --> enhanced lipolysis

Acetylcholinesterase inhibition

Increased availability of acetylcholine at the NMJ

Enhancing the release of Ca2+ from the sarcoplasmic reticulum

Increased contractility of skeletal muscle

Inhibition of PTP1B

Potentiation of insulin signaling and other growth promoting cascades (unknown tissue specificity)

High Tower Pharmacology

Sunday, September 2, 2012

Future Pharmacy: Amentoflavone