Enzyme found in pineapple stems stimulates the gut to produce enkephalin, an opioid peptide normally released by the brain (images: Peptides / Pixabay)
Published on 05/12/2021
By Maria Fernanda Ziegler | Agência FAPESP – Bromelain, an enzyme extracted from the stem and fruit of the pineapple (Ananas comosus), has long been known to the pharmaceutical industry, but its pain relief mechanism has only now been fully revealed.
In a study supported by FAPESP as part of a Thematic Project, researchers at the Medical School of the Federal University of São Paulo (EPM-UNIFESP) in Brazil have discovered that bromelain triggers the release of proenkephalin, the precursor of enkephalin, which is considered an endogenous opioid and is present in the gastrointestinal tract.
Enkephalin is a naturally occurring peptide that has potent painkilling effects and is released by neurons in the brain and central nervous system. It is derived from the proenkephalin precursor protein via specific proteases (enzymes that breakdown proteins and peptides) present in brain tissue and is an important route for pain control. Like morphine, enkephalin acts on opioid receptors.
“We set out to investigate exactly how bromelain acts as a painkiller when ingested. We knew it cannot enter the bloodstream because it would cause a violent hypotensive shock leading to death [which is why bromelain is never administered intravenously for therapeutic purposes]. Therefore, the effect would have to occur via a different mechanism confined to the surface of the intestine,” said Luiz Juliano, Full Professor (retired) at UNIFESP and one of the authors of an article on the study published in the journal Peptides.
The presence of proenkephalin in regions other than the brain, including the gastrointestinal tract, was discovered approximately five years ago, according to Juliano. “We put these two facts together and proved through in vivo experiments that the contents of gut tissue participate in pain control,” he told Agência FAPESP.
In experiments with mice, the researchers found that when bromelain is ingested, it releases enkephalin by digesting proenkephalin from the small intestine. This enkephalin enters the bloodstream and has a peripheral analgesic effect.
The discovery suggests that further research should be able to reveal how enzymes in the bolus and gut microbiota interact with the intestinal wall to release bioactive substances.
Indigenous peoples in the New World knew about pineapple’s analgesic effect centuries ago. The first Europeans to reach the Americas reportedly took home specimens used to dress wounds and relieve pain.
Centuries later, scientists discovered that bromelain can treat not only pain but also inflammation and breaks down proteins, leading to the development of many products by the pharmaceutical and food industries as an aid to digestion and pain relief and to heal skin injuries and tenderize meat (read more at agencia.fapesp.br/28299).
Despite the commercial success of bromelain, the link between its analgesic effect and its role in the gut microbiota was poorly understood. The EPM-UNIFESP research group’s studies in mice show that the interaction between bromelain and the intestinal mucosa releases enkephalin, which relieves pain.
“Enkephalin produced in the intestine acts mainly in the periphery of the organism, where it may have anti-inflammatory properties,” Juliano said.
According to the Peptides article, oral administration of bromelain to mice reduced levels of proenkephalin in the jejunum, part of the small intestine, and increased levels of enkephalin in the bloodstream.
The analgesic effect of bromelain administered at a dose of 3 mg/kg peaked after three hours. “The fact that there’s a limit is intriguing,” Juliano said. “The effect is constant up to a certain dose of bromelain, but then as the dose is increased, the effect diminishes until no analgesic action can be detected. We found that this is due to hydrolysis of enkephalin, probably because commercially available bromelain is impure and contains other proteases.”
The process can be elucidated by understanding how bromelain breaks down proenkephalin. Bromelain extracted from the pineapple stem mainly breaks down proteins after paired sequences of basic amino acids arginine and lysine. Proenkephalin contains seven sequences of enkephalin flanked by these basic amino acid pairs.
The researchers treated chemically synthesized fragments of proenkephalin with bromelain and found that enkephalin was released when the amino acids were hydrolyzed.
According to Juliano, bromelain’s preference for arginine and lysine, first observed in a study published in August 1998, has similarities to the action mechanism of the protease PC2, the main activating enzyme (convertase) that cleaves proenkephalin to produce enkephalin in the brain. The EPM-UNIFESP group also participated in the study described in 1998.
“Bromelain’s preference for arginine and lysine makes it function like the specific enzymes in the brain that produce enkephalin,” Juliano said. “Our studies show that the effects of bromelain and morphine are similar, which can be explained by the fact that morphine acts on the same receptors as enkephalin. However, the analgesic neurotransmitter is produced endogenously in our organism.”
Bromelain may also prove useful in further elucidation of brain-intestine interactions, he added.
“This study not only explains bromelain’s action mechanism but also inspires us to investigate the interaction between the gut microbiota and the lining of the intestine, especially its enzymatic elements. In addition to food, we’ll also analyze the microbiota and its products, which undoubtedly produce proteolytic enzymes. These may trigger significant physiological responses such as pain and inflammation, as well as immune responses,” Juliano said.
The article “Enkephalin related peptides are released from jejunum wall by orally ingested bromelain” (doi: 10.1016/j.peptides.2019.02.008) by Paulo Eduardo Orlandi-Mattos, Rodrigo Barbosa Aguiar, Itabajara da Silva Vaz Junior, Jane Zveiter Moraes, Elisaldo Luiz de Araujo Carlini, Maria Aparecida Juliano and Luiz Juliano can be read at www.sciencedirect.com/science/article/pii/S0196978119300245.