Introduction

In contrast to most trace elements, zinc is fairly evenly distributed throughout animal tissues, the highest concentrations of zinc are found in epidermal tissues (skin and hair).

Metabolism

Zinc is absorbed principally throughout the small intestine of monogastric animals, little from the stomach, even the cecum and colon contribute to zinc absorption when the small intestine is impaired. Within the intestinal mucosa cell, zinc transfer is regulated by metallothionein, a metal-binding protein produced by the liver. Metallothionein synthesis is influenced by both dietary and plasma zinc, thus a central role in zinc homeostasis (Cousins).

Many dietary factors reduce zinc absorption; phytate, calcium plus phytate, phosphorous, copper, cadmium. Dietary fibre increased zinc absorption that had been suppressed by dietary phytate (Hayashi et al, 2001).

Absorption

Zinc absorption is increased by casein, liver extract, corn oil, chelating agents and vitamin D, zinc absorption is related to physiological demand. In plasma, zinc may be loosely bound to albumin and may be passed to a metallo-ligand complex with amino acids or EDTA as non-protein ligands. Zinc moves into and out of cells by transport proteins.

Storage

Zinc is widely distributed in the body of the horse, however limited storage capacity in a form that can be mobilised to prevent deficiency. The system metabolises zinc very rapidly, it doesn’t hold much useful zinc in reserve (Cousins, 1999). Metallothionein is a major storage form of zinc in the liver and is mobilised during a need. Zinc is found in high concentrations in glandular organs.

Physiological functions

The primary role of zinc is associated with enzymes both as part of the molecule or as an activator.

• Zinc often stabilises the quaternary structures of RNA, DNA and ribosomes
• Over 1000 zinc proteins known, some functions related to cell replication and differentiation (Maret, 2002)
• Zinc as enzyme systems largely involve nucleic acid metabolism, protein synthesis and carbohydrate metabolism
• In rapidly growing tissues, zinc deficiency greatly reduces synthesis of DNA, RNA and protein, hence, impairs cellular division, growth and repair. (Ho and Ames, 2002)
• Zinc proteins involved in transcription and translation of the genetic material
• A sequence of amino acids tightly bound with zinc resemble a sausage or finger. This is called a "zinc finger". Zinc is required for binding of protein to DNA
• The DNA-binding proteins with "zinc fingers" regulates genetic expression
• DNA binding proteins that contain "zinc fingers" include transcription factors that bind to respective hormones and receptors such as for retinoic acid (RXR, RAR) and Vitamin D (VDR)

Hormones

Zinc plays a role in production, storage, and secretion of individual hormones as well as effectiveness of receptor sites. Zinc is important regarding both the stallion and mares reproductive capacity. For the mare significantly after giving birth through the lactation period.

Deficiency

Zinc deficiency in foals affects growth rate and causes cutaneous lesions on the lower extremities and alopecia. Skin lesions first appear in the hoof area (6–7th week) to face lesions (70–80 days), skin removes itself in small sheets. Contributing factors can be parasitic infections, geophagia, and chronic infectious diseases. Depressed reproductive ability and impaired immunity are often encountered.