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=Monocrotaline= Monocrotaline (MCT) is a pyrrolizidine alkaloid that is present in plants of the Crotalaria family. This family can synthesise MCT out of amino acids and can cause liver, lung and kidney damage in various organisms. Initial stress factors are released intracellular upon binding of MCT to BMPR2 receptors and elevated MAPK phosphorylation levels are induced, which can cause cancer in Homo sapiens. MCT can be detoxified in rats via oxidation, followed by glutathione-conjugation and hydrolysis.

Origin
MCT occurs in the seeds of certain species of the family Crotalaria, for example Crotalaria spectabilis and Crotalaria mucronata. MCT is a chemical with pesticide properties and therefore serves as a defence mechanism to fend off predators. However, it can also lead to the poisoning of mammals and birds.

The butterfly Utetheisa ornatrix, a family of tiger moths, also benefits from MCT by using  it as protection. The larvae of the butterfly feed almost exclusively on Crotalaria seeds, where MCT is accumulated in their bodies. In this way, they are protected from predators such as spiders for the rest of their lives (even after pupation as butterflies).

Toxicity
MCT is an acute toxic substance. The toxicity of MCT is dose-dependent, it can harm both organs and genetic material (genotoxicity). The organs that will be targeted are the liver (hepatotoxicity), the kidneys (nephrotoxicity) and the lungs (pneumotoxicity). MCT falls into Category 3 toxicity for oral ingestion and Category 2 toxicity for carcinogenicity according to the European Chemicals Agency (ECHA).

Studies concluded that the ingestion of MCT will cause centrilobular necrosis, pulmonary fibrosis and increase in blood urea nitrogen. These conclusions are based on the models that were used during these studies as these effects were caused in rats instead of humans. During the studies it was also concluded that mice are more resilient to MCT than rats, meaning that more mice survived the experiments than rats.

Biosynthesis of monocrotaline
Plants of the Crotalaria family can synthesise MCT out of two precursors during biosynthesis; monocrotalic acid (MCA) and retronecine. Both molecules are derived from amino acids, present in the plant species.

During the biosynthesis of monocrotalic acid, L-isoleucine is used as a precursor to form the MCA. L-Isoleucine is therefore synthesised out of L-threonine via the following reaction:

L-threonine is deaminated to form α-ketobutyrate (1: threonine deaminase ), which is coupled to pyruvate and forms 2-aceto-2-hydroxybutyrate and free carbon dioxide (2: acetoacetate synthase ). The keton of 2-aceto-2-hydroxybutyrate is then reduced by NADPH and H+ to form 2,3-dihydroxy-3-methylvalerate and is subsequently dehydrated to form 2-keto-3-methylvalerate (3: ketol-acid reductoisomerase and 4: dihydroxy-acid dehydratase respectively ). The final step in the biosynthesis of L-isoleucine is transamination by glutamate, which is converted to α-ketoglutarate. This step is catalysed by aminotransferase (5) and yields the L-isoleucine that is used in the biosynthesis of MCA.

L-isoleucine reacts further with one or several unknown precursor(s) to synthesise MCA via the following reaction: Retronecine, the other precursor that is needed to synthesise MCT, is also synthesised out of an amino acid, namely L-arginine. The biosynthesis of retronecine out of L-arginine proceeds as follows:

Biosynthesis of retronecine starts out with L-arginine, which is converted to putrescine by decarboxylation (1: arginine decarboxylase) and two hydrolysis reactions (2: agmatine iminohydrolase and 3: N-carbomoylputrescineamidohydrolase, respectively).

The obtained intermediate is then converted to spermidine by addition of a n-propylamine group from decarboxylated S-adenosylmethioninamine (4: spermidine synthase). The intermediate reacts with its precursor putrescine to form the symmetric homospermidine by splitting of 1,3-diaminopropane (5: homospermidine synthase).

Oxidation (likely catalysed by 6: copper-dependent diamine oxidases) to 4,4’-iminodibutanal results into the cyclization of pyrrolizidine-1-carbaldehyde, which is reduced to 1-hydroxymethyl pyrrolizidine (likely catalysed by 7: alcohol dehydrogenase). To form the final product retronecine, 1-hydroxymethyl pyrrolizidine is desaturated and hydroxylated respectively by unknown enzymes.

MCA and retronecine are then condensed to form MCT via an unknown mechanism:

Biotransformation of monocrotaline
MCT is detoxified in rats by the liver via divergent biotransformation reactions. These reactions proceed as follows:

In Rats, MCT is first oxidised by the biotransformation enzyme cytochrome P450 (CYP) to form dehydro MCT. In this phase 1 reaction a double carbon-carbon bond is introduced out of a single carbon-carbon bond.

After the phase 1 reaction, the oxidised intermediate can either undergo hydrolysis to form monocrotalic acid and dihydropyrolizine or perform group transfer with glutathione to form MCA and a glutathione-conjugated dihydropyrolizine (GS-conjugation). These metabolites are more polar than MCT and could therefore be more easily excreted by the kidneys, which results in less exposure from MCT to the liver. The phase 2 reactions are thus classified as the detoxifying reactions during the biotransformation of MCT in rats.

Note that the biotransformation routes may differ based on the studied organism.

Mechanism of action
MCT aggregates on and activates the calcium-sensing receptor (CaSR) of pulmonary artery endothelial cells to trigger endothelial damage and, ultimately, induces pulmonary hypertension 1. MCT binds to the extracellular domain of the CaSR (calcium-sensing receptor). Thereby, the assembly of CaSR is enhanced and triggers the mobilisation of calcium signalling, and damages pulmonary artery endothelial cells. In addition, MCT strengthens this effect by binding to the bone morphogenetic protein receptor type II (BMPR2), which is a transmembrane receptor. BMPR2 inhibition occurs which in turn induces a blockade of BMPR1 receptor activation via phosphorylation. Inhibiting this process disturbs cell differentiation processes and ossification. Interference with these receptors induce pulmonary arterial hypertension2.

MAPK is a mitogen activated protein kinase that gets activated upon BMPR2 activation. The protein kinase in turn phosphorylates p38 via a reinforced cascade of intracellular signals. It also activates p21 which has a regulating role in the cell cycle. However, MCT administration inhibits this process via a blockade of BMPR2. Cytokines such as TNF-α are released which cause activation of inflammation mechanisms, attracting neutrophils among others. Furthermore, inducible nitric oxide synthases (iNOS) are upregulated upon MCT induced cellular stress, whereas endothelial NOS (eNOS) gets downregulated. The cytokine TGF-β (also released by macrophages via chemotaxis during inflammation reactions in a positive feedback loop with TNF-α) is a transforming growth factor that is upregulated as a result of iNOS increasement, contributing to pulmonary artery proliferation. Increased levels of iNOS also stimulate caspase-3 activity which increases apoptosis levels.

Experiments
Rats that were treated with MCT had a repressed growth compared to their control groups. A significant difference in body weight was apparent after 14 days of MCT supplement treatment. After 28 days the rats had 67% body fat compared to the control group. The decrease in body fat was accompanied with the decrease in water consumption. Even though the rats weighed less than their control group, their lungs weighed more than then their control counterparts. Pulmonary changes that occurred because of treatment with MCT were linked to the disposition of 5-hydroxytryptamine (5HT) in the isolated lungs.

The weight ratio between liver and body was unchanged after the treatment with MCT. The activity of plasma glutamic pyruvic transaminase (GPT) remained also unchanged until the 28th day. After day 28 the activity was elevated compared to the control group.

The weight ratio between kidney and body were slightly higher after the treatment with MCT, during day 7 to 14. This effect did not last however. It was speculated that this early effect occurred because of changes in body weight, since the absolute kidney weight did not increase. The urea nitrogen concentration in blood did increase after 28 days. After 28 days the concentration of p-aminohippuric acid (PAH) in the kidneys was lower than that of the control group. The concentration of  tetraethylammonium (TEA) was on the other hand higher than that of the control group.