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- InChI, InChIKey, MOSFIJXAXDLOML-UHFFFAOYSA-N, smiles, IUPAC Name.

ChemSub Online: The Free Web Portal and Information System on Chemical Substances from around the World: Glossary and Acronyms - Letter P.
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ChemSub Online: The Free Web Portal and Information System on Chemical Substances from around the World: Glossary and Acronyms - Letter P.
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Pa:
Pascal unit(s).
 
PAF:
Platelet Activating Factor.

Is an acetylated derivative of glycerophosphorylcholine, so called as one of its main effects is to induce platelet aggregation.

Metabolism:
PAF is not stored in cells, it is synthesized, in response to various stimulations, by platelets, leukocytes, endothelial cells and mast cells.

The inactive precursor is 1-O-alkyl-2-acyl-glycerophosphocholine which, under the influence of A2 phospholipase, is hydrolyzed into an acid which is generally arachidonic acid and in lyso-PAF. The lyso-PAF is then acetylated by an acetyltransferase in active PAF. PAF is inactivated by desacetylation by an acetylhydrolase which transforms it into lyso-PAF, itself transformed by an acyltransferase into the inactive precursor, 1-O-alkyl-2-acyl-glycérophosphcholine. The inhibition of acetylhydrolase increases the tissue concentration of PAF.

Effects of PAF:
PAF stimulates receptors coupled to G proteins which activate phospholipases C and A2, inducing the formation of diacylglycerol, inositolphosphate and arachidonic acid (A2).

- It elicits platelet aggregation. An intravenous injection in animals elicits the formation of a platelet clot, accompanied by thrombocytopenia.
- It induces leukocytes aggregation and is involved in chemotaxis of eosinophils, neutrophils and monocytes.
- It is a powerful vasodilator able to cause an important arterial hypotension, it increases vascular permeability and movement of liquid out of the vasculature.
- It constricts directly, or indirectly by release of autacoids, the majority of smooth muscles, except vascular fibers. It constricts digestive, uterine, bronchial fibers. Moreover it increases for a long duration the reactivity of bronchi to other bronchoconstrictors. PAF induces gastric ulcerations.
- It decreases renal blood flow rate, diuresis and sodium excretion.
- It probably is involved in ovulation and egg: implantation could be inhibited by PAFantagonists.
- It could induce uterine contractions during labor and its antagonists could delay parturition.
- It increases vascular permeability, edema and cellular chemotaxis and takes part in inflammatory reactions.

Drugs and PAF:
Most of the effects of PAF are regarded as adverse but . One could try to reduce them by:

1. inhibition of bio synthesis and release of PAF: but there does not exist at present of specific drug exerting these effects.
2. inhibition of PAF receptors: compounds which could have antagonist effects on PAF receptors are terpenes, isolated from Ginkgo biloba such as ginkgolide B. Extracts of Ginkgo biloba were largely prescribed for preventive and curative treatment of cognitive and vasomotor disturbances.

On the whole, one does not have drugs today able to inhibit specifically PAF biosynthesis or effects.
 
PBK:
Physiologically Based Kinetics.
 
PBT:
Persistent Bioaccumulative and Toxic.

PBT pollutants are chemicals that are toxic, persist in the environment and bioaccumulate in food chains and, thus, pose risks to human health and ecosystems. The biggest concerns about PBTs are that they transfer rather easily among air, water, and land, and span boundaries of programs, geography, and generations.
 
PEC:
Predicted Environmental Concentration.
 
Persistence:
A chemical that resists degradation processes and is present in the environment for a long time. Specific criteria have been established in Persistent Organic Pollutant (POP) protocols, in the TGD and in REACH (PBT/vPvB). In the latter persistent (P) and very persistent (vP) refers to chemicals that have degradation half-lives above certain trigger values in surface water, sediment or soil.
 
pH:
Potential hydrogen -logarithm (to the base 10) of he hydrogen ion concentration {H+}.
 
Photolysis:
Chemical decomposition or degradation induced by light or other radiant energy.

Direct photolysis in natural water involves the transformation of a chemical resulting from the direct absorption of a solar photon. Indirect photolysis in natural water sometimes involves the transformation of a chemical due to energy transfer from naturally occurring photosensitizers in electronically excited triplet states. However, indirect photolysis more often involves the transformation of a chemical due to reactions with transient oxidants such as hydroxyl radicals, molecular oxygen in a singlet electronic state, and peroxy radicals. Indirect photolysis is an important transformation process for chemicals in the gaseous state in air.
 
pKa:
-logarithm (to the base 10) of the acid dissociation constant.
 
pKb:
-logarithm (to the base 10) of the base dissociation constant.
 
PNEC:
Predicted No-Effect Concentration.

Concentration of the substance below which adverse effects in the environmental sphere of concern are not expected to occur.
An ecotoxicological measure for multiple species systems. It can be defined as the concentration below which a specified percentage of species in an ecosystem are expected to be protected.
 
PNECwater:
Predicted no effect concentration in water.
 
PNOC:
Particulates Not Otherwise Classified.

Particulates not otherwise classified is a term defined by the ACGIH (American Conference of Governmental Industrial Hygienists). It is used to describe particulates for which there is no evidence of specific toxic effects such as fibrosis or systemic effects. These material are not to be considered inert, however, and can produce general toxic effects depending on the airborne concentration.
 
Polymère:
Une substance constituée de molécules se caractérisant par une séquence d'un ou de plusieurs types d'unités monomères et contenant une simple majorité pondérale de molécules contenant au moins trois unités monomères liées par liaison covalente à au moins une autre unité monomère ou une autre substance réactive et constituée de moins qu'une simple majorité pondérale de molécules de même poids moléculaire.
Ces molécules doivent former une gamme de poids moléculaires au sein de laquelle les différences de poids moléculaire sont essentiellement attribuables à la différence dans le nombre d'unités monomères.
Au sens de la présente définition, on entend par "unité monomère" la forme réagie d'un monomère dans un polymère.
 
Polymer:
A substance consisting of molecules characterized by the sequence of one or more types of monomer units and comprising a simple weight majority of molecules containing at least three monomer units which are covalently bound to at least one other monomer unit or other reactant and consists of less than a simple weight majority of molecules of the same molecular weight.
Such molecules must be distributed over a range of molecular weights wherein differences in the molecular weight are primarily attributable to differences in the number of monomer units.
In the context of this definition a "monomer unit" means the reacted form of a monomer in a polymer.
 
Polymerize, Polymerization:
Polymerization is the process of forming a polymer by combining large numbers of chemical units or monomers into long chains. Polymerization can be used to make some useful materials. However, uncontrolled polymerization can be extremely hazardous. Some polymerization processes can release considerable heat, can generate enough pressure to burst a container or can be explosive. Some chemicals can polymerize on their own without warning. Others can polymerize upon contact with water, air or other common chemicals. Inhibitors are normally added to products to reduce or eliminate the possibility of uncontrolled polymerization.
 
ppm:
parts per million.
 
PPV:
Predictive Positive Value.
 
PRA:
Probabilistic Risk Assessment.
 
Primary biodegradation:
The structural change (transformation) of a chemical substance by microorganisms resulting in the loss of the original chemical identity.
 
Priority List:
(EU) In 1993 the Council adopted Council Regulation (EEC) 793/93 or the Existing Substances Regulation (ESR), thereby introducing a comprehensive framework for the evaluation and control of "existing" chemical substances.

Article 8 of the Regulation states that the Commission, in consultation with the Member States, will regularly draw up lists of priority substances which require immediate attention because of their potential effects to man or the environment. The Commission and Member States utilise the information collected during step I of the regulation (Data Collection) as a basis for selecting priority substances.
Since 1994, 4 such priority lists have been published with a total of 141 substances.
 
PSL:
Priority Substances List.

The Canadian Environmental Protection Act, 1999 (CEPA 1999, section 76) requires the Ministers of the Environment and of Health to establish a Priority Substances List (PSL) that identifies substances to be assessed on a priority basis to determine whether they are toxic (as defined under Section 64 of the Act) and pose a risk to the health of Canadians or to the environment. Assessments of substances placed on the PSL are the shared responsibility of Environment Canada and Health Canada. A Priority Substance may be a chemical, a group or class of chemicals, effluents or wastes.

Assessments of the first 44 substances placed on the list (PSL1) were completed by February 1994. Following the recommendations of a multi-stakeholder Expert Advisory Panel, 25 substances were added to the list (PSL2).
 
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