Components of phosphatidylserines:
Blue, green: variable fatty acid groups
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via apoptotic mimicry.
Phosphatidylserine is a phospholipid (more specifically a glycerophospholipid). It consists of two fatty acids attached in ester linkage to the first and second carbon of glycerol and serine attached through a phosphodiester linkage to the third carbon of the glycerol.
Phosphatidylserine coming from plants and phosphatidylserine coming from animals differ in fatty acid composition.
Phosphatidylserine(s) are actively held facing the cytosolic (inner) side of the cell membrane by the enzyme flippase. However, when a cell undergoes apoptosis, phosphatidylserine is no longer restricted to the cytosolic side by flippase. Instead scramblase catalyzes the rapid exchange of phosphatidylserine between the two sides. When the phosphatidylserines flip to the extracellular (outer) surface of the cell, they act as a signal for macrophages to engulf the cells.
Phosphatidylserine plays a role in blood coagulation (also known as clotting). When circulating platelets encounter the site of an injury, collagen and thrombin -mediated activation causes externalization of phosphatidylserine (PS) from the inner membrane layer, where it serves as a pro-coagulant surface. This surface acts to orient coagulation proteases, specifically tissue factor (TF) and factor VII, facilitating further proteolysis, activation of factor X, and ultimately generating thrombin.
In the coagulation disorder Scott syndrome, the mechanism in platelets for transportation of PS from the inner platelet membrane surface to the outer membrane surface is defective. It is characterized as a mild bleeding disorder stemming from the patient's deficiency in thrombin synthesis.
Phosphatidylserine is biosynthesized in bacteria by condensing the amino acid serine with CDP (cytidine diphosphate)-activated phosphatidic acid. In mammals, phosphatidylserine is produced by base-exchange reactions with phosphatidylcholine and phosphatidylethanolamine. Conversely, phosphatidylserine can also give rise to phosphatidylethanolamine and phosphatidylcholine, although in animals the pathway to generate phosphatidylcholine from phosphatidylserine only operates in the liver.
The average daily phosphatidylserine (PS) intake from diet in Western countries is estimated to be 130 mg. PS may be found in meat and fish. Only small amounts of PS can be found in dairy products or in vegetables, with the exception of white beans and soy lecithin.
Table 1. PS content in different foods. Soy products are not in this table, because commercial PS is made by enzymatically converting soy phosphatidylcholine (lecithin) to phosphatidylserine, rather than purifying phosphatidylserine from soy.
|Food||PS Content in mg/100 g|
|Offal (average value)||305|
|Chicken leg, with skin, without bone||134|
|Chicken breast, with skin||85|
|Turkey leg, without skin or bone||50|
|Turkey breast without skin||45|
|Whole grain barley||20|
|European pilchard (sardine)||16|
|Cow's Milk (whole, 3.5% fat)||1|
A panel of the European Food Safety Authority concluded that a cause and effect relationship cannot be established between the consumption of phosphatidylserine and "memory and cognitive functioning in the elderly", "mental health/cognitive function" and "stress reduction and enhanced memory function". The reason is that bovine brain cortex- and soy-based phosphatidylserine are different substances and might, therefore, have different biological activities. Therefore, the results of studies using PS coming from different sources cannot be generalized.
In May, 2003 the Food and Drug Administration gave "qualified health claim" status to phosphatidylserine thus allowing labels to state "consumption of phosphatidylserine may reduce the risk of dementia and cognitive dysfunction in the elderly" along with the disclaimer "very limited and preliminary scientific research suggests that phosphatidylserine may reduce the risk of cognitive dysfunction in the elderly." According to the FDA, there is a lack of scientific agreement amongst qualified experts that a relationship exists between phosphatidylserine and cognitive function.
More recent reviews have suggested that the relationship may be more robust, though the mechanism remains unclear. Some studies have suggested that whether the phosphatidylserine is plant or animal derived may have significance, with the FDA's statement applying specifically to soy-derived products.
Traditionally, PS supplements were derived from bovine cortex (BC-PS). However, due to the risk of potential transfer of infectious diseases such as bovine spongiform encephalopathy (or "mad cow disease"), soy-derived PS supplements have been used as an alternative. Soy-derived PS is designated Generally Recognized As Safe by the FDA. A 2002 safety report determined supplementation in elder people at a dosage of 200 mg three times daily to be safe. Even so, concerns about the safety of soy products persist, and some manufacturers of PS use sunflower lecithin instead of soy lecithin as a source of raw material for PS production.