- Slides: 8
Macrominerals Magnesium Dr. Sherin Bakhashab
Magnesium Sources: Leafy green vegetables (containing chlorophyll). Requirements: 1. The RDA for adult man is 350 mg/ day 2. More magnesium is required for pregnancy and lactating 450 mg/day.
Distribution 1. Total amount of magnesium in the body is about 25 g. 2. It occurs in bones, muscles and nervous system. 3. Inside the cells, it is concentrated in the mitochondria, where it is part off many enzymatic systems responsible for energy. 4. The distribution of Mg is uneven as it replaces Ca to some extent and this depends mainly on the amount of Ca available.
Blood Magnesium 1. Plasma magnesium is 2 -3 mg/dl. 2. The concentration of Mg in red blood cells is 3 times greater than plasma. 3. Factors affecting plasma Mg: a) Aldosterone: It decreases plasma Mg level by increasing its excretion by the kidney. b) Parathyroid hormone: increases plasma Mg level by inducing the mobilization of Mg from bones. c) Kidney function: Renal failure leads to hypermagnesemia, due to failure of Mg exertion in the urine.
Absorption of Magnesium 1. Most food contain good amount of Mg. 2. Thus Mg deficiency does not rise from inadequate supply but may due to some metabolic disturbances affecting its absorption or increasing its excretion. 3. Normally, 40% of the ingested Mg is absorbed from the upper small intestine. 4. Its absorption increases by vitamin D and parathyroid hormone (PTH). 5. PTH decreases Mg excretion in urine.
Functions of Magnesium 1. It enters in the structure of skeleton (bones and teeth). 2. It is important for muscle contraction, nerve impulse transmission. 3. It activates many enzymes, e. g. kinases, phosphatases, phosphorylases and some decarboxylase.
Mg as an activator of enzyme activity § Restriction nucleases, ligases, and topoisomerases are among many enzymes that are stimulated by divalent Mg. § Mg plays a role in enzymatic reactions by 2 ways: 1. An enzyme bind Mg-substrate complex: In this case, the enzyme interacts mainly with the substrate and shows weak interaction with Mg+2, e. g. kinases. 2. An enzyme binds directly to Mg+2 : This leading to conformational change of the enzyme and its activation.
Excretion of Magnesium § The excretion of Mg is mainly through the feces (70%), it is mostly unabsorbed Mg phosphate. § A small amount of Mg (30%) is excreted through the urine. § The renal output is the most important factor for plasma Mg regulation.