APPENDIX C—METALLOTHIONEIN

Introduction

Metallothionein (MT) proteins play an important role in mental health. Poor MT function has been associated with ADHD, autism, schizophrenia, Alzheimer’s disease, and Parkinson’s disease. MT proteins perform a myriad of vital functions including the following processes:

image   Early brain cell development

image   Powerful antioxidant capability

image   Detoxification of mercury and other toxic metals

image   Reduction of inflammation after injury or illness

image   Enhanced efficiency of the intestinal and blood-brain barriers

image   Development and functioning of the immune system

image   Delivery of zinc to cells throughout the body

image   Homeostatic control of zinc and copper levels in blood

image   Prevention of yeast overgrowth in the intestines

image   Regulation of stomach acid pH

image   Taste discrimination by the tongue

image   Protection of enzymes that break down casein and gluten

image   Zinc signaling in brain cells

image   Regulation of tumor suppression genes

image   Transcription factor regulation

The Metallothionein Family of Proteins

Metallothioneins are short, linear, cysteine-rich proteins composed of between 61 and 68 amino acids. All human MTs contain 20 cysteines and have an “S” configuration with extraordinary metal binding capability. There are four varieties of metallothionein proteins. MT-I and MT-II are found throughout the body, and their functions include regulation of zinc and copper levels, development of neurons and synaptic connections, enhancement of immune function, and protection against toxic metals. MT-III is a necessary factor in the pruning and growth-inhibitory phases of brain cell development. MT-IV regulates stomach acid pH and enables taste discrimination by the tongue.

Synthesis of MT proteins involves genetic expression of thionein (induction) followed by loading of thionein with metal atoms. MT-I and MT-II take on seven zinc atoms, while MT-III typically contains four copper atoms and 3 zinc atoms. MT proteins are generated in response to injury, illness, emotional stress, or exposure to toxic metals. They represent a major antioxidant system in the body.

MT proteins are found at high levels in four brain areas: hippocampus, amygdala, pineal gland, and cerebellum. The hippocampus is essential to cognition, speech, learning, memory, and behavioral control. The amygdala has a role in emotional memory and socialization. The pineal gland produces melatonin that assists sleep. The cerebellum enables smooth physical movements. Weakened MT function could result in problems in any of these areas.

Brain Development

In infancy, the brain has a high population of small, densely packed neurons. MT-III plays an important role in pruning of brain neurons during early development, which enables the remaining brain cells to grow and develop synaptic connections. In addition, MT-III is the primary inhibitory factor that stops the growth process when brain cells reach optimal size. An early MT-III dysfunction would be expected to result in the following:

image   Incomplete pruning

image   Areas of densely packed undeveloped neurons

image   Increased brain volume and head diameter

All of these phenomena have been reported in autism spectrum disorders. This understanding has led to MT-Promotion therapies aimed at completion of brain development in children. These therapies are also under development for Alzheimer’s disease since extremely low MT levels have been observed in this disorder.

Detoxification of Heavy Metals

Metallothioneins are heavy metal magnets. They bind mercury, lead, cadmium, and other toxic metals tightly and render them relatively harmless. Deficiencies in metallothionein functioning would, therefore, be expected to lead to an increased burden of these dangerous substances. MT proteins work in tandem with GSH and selenium. Metal atoms are transferred into thionein by reduced GSH to form Zn7MT. However, glutathione disulfide (GSSG) enables the release of zinc in exchange for another atom, for example, mercury, cadmium, lead, or copper. The cellular redox state of GSH determines the direction of zinc transfer. For example, GSH efficiently binds to toxic mercury but has a limited capacity. When more than 10% of reduced GSH has been converted to GSSG (oxidized GSH), the GSSG activates MT to enable its participation in sequestering toxic metals. In essence, GSH is the first defense against mercury and other heavy metals, and MT joins the fray after GSH levels have been significantly depleted. Selenium increases the kinetics of mercury transfer into MT by about 50%. Optimal protection against toxic metals requires proper amounts of GSH, MT, and selenium, and I refer to them as The Three Musketeers.

Intestinal and Blood-Brain Barriers

MT-I and MT-II are present in very high concentrations in intestinal mucosa, forming a barrier to penetration of mercury, lead, and other toxins into the portal blood stream. With respect to toxic metals, the expression leaky gut often means a failure of MT to function normally. In healthy persons, toxic metals in the diet are sequestered in mucosal MT, which is sloughed off every 5 to 10 days to be left harmlessly in the stool. It is impossible to avoid significant exposures to toxic metals in normal living, and the MT system is needed every day. For example, the average amount of mercury in a typical adult diet is about 20 micrograms/day, with higher amounts for high seafood diets. The average amount of mercury entering the body from breathing (in the USA) is one microgram/day.

MT proteins are in high concentration at the blood-brain barrier (BBB) and represent the primary protection against toxic metals from entering the brain. In addition, MT proteins within the brain assist in sequestering any toxins that penetrate the BBB. It has been estimated that in healthy adults, 90% of mercury in the diet is prevented from entering the portal blood stream that flows to the liver. In the liver, MT, GSH, and other antioxidants bind to about 90% of the mercury that has penetrated the intestinal barrier. The MT in the BBB is believed to be about 90% efficient in stopping mercury’s access to the brain. In summary, less than one ingested mercury atom (or compound) in 1,000 is able to enter the brain of healthy persons. However, if MT function is weak or disabled, toxic metals can wreak havoc in the brain by altering neurotransmitter synthesis, destroying myelin, producing inflammation, increasing oxidative stress, and, in some cases, killing brain cells. Two studies have indicated MT levels are less than one-third of the normal concentration in Alzheimer’s patients, and this may be a factor in the relentless death of brain cells in this disease.

Metallothionein and the GI Tract

The highest concentrations of MT proteins in the body are in the GI tract. An important role of MT in the intestines is the donation of zinc for synthesis of the enzymes carboxypepidase A and aminopepidase, which are needed to break down casein, gluten, and other proteins from food. Zinc is also required for proper functioning of dipeptidyl peptidase-IV, which breaks down gliadin, casomorphins, and other proline-containing proteins. A significant impairment in MT function could cause incomplete breakdown of casein, gluten, casomorphins, etc., which could result in severe food allergies. In my experience, about 85% of the autism population reports major lessening of symptoms after a gluten-free/casein-free diet.

MT has other important roles in the GI tract. For example, MT is an important defense mechanism against intestinal inflammation and diarrhea. In addition, MT proteins kill Candida and tend to prevent yeast overgrowth. Stomach parietal cells are rich in MT-IV proteins that promote formation of hydrochloric acid (HCl). MT-IV on the surface of the tongue enables taste discrimination.

Metallothionein and Immune Function

The importance of metallothionein in immune function has been known for more than 20 years. MT proteins are the primary vehicle for delivery of zinc to cells, and zinc deficiency can severely impair the immune system. In animal studies, reduced levels of MT and zinc during gestation resulted in atrophy of thymic and lymphoid tissues and greatly weakened immune response to infections. Experiments involving knockout mice (a strain of rodents with one or more genes removed—in this case an MT gene) showed severely impaired immunity.

Weak MT activity can result in a premature transition from cell-mediated immunity to humoral response and can result in a decreased amount of circulating T cells. MT also enhances immune function through its role as an efficient scavenger of free radicals. When the body is under attack by bacteria or viruses, macrophages and neutrophils work overtime to destroy the invaders. Once they have engulfed and killed an intruder, excess hydrogen peroxide is left behind, and MT is effective in mopping up this toxic oxidizing chemical.