METABOLISM OF PHOSPHOROUS

2. Phosphorous Metabolism
Gandham. Rajeev

3.  Phosphorous is a widely distributed in the body
 The human body contains about 1 kg of phosphorous
 Body distribution:
 About 80% of phosphorous is found in bones & teeth in
combination with calcium
 About 15% of phosphorous is present in soft tissues
 In the soft tissues most phosphate is organic, as a
component of phospholipids, phosphoproteins, nucleic
acids & nucleoproteins

4.  About 1% is found in ECF
 In the ECF phosphorous is present in the inorganic
form,
 About 5g in brain & 2g in blood is found
 Phosphorous is present in the form of organic &
inorganic

5. Dietary sources and RDA
 The food rich in calcium is also rich in phosphorous, i.e.
milk, cheese, beans, eggs, cereals, fish and meat
 Milk is good source of phosphorous, which contains about
100 mg/dl of phosphorous
 The daily requirement of phosphorous is about 800 mg/day
 During pregnancy and lactation 1,200 mg/day is required

6. Biochemical functions
 Phosphorous is essential for formation of bones & teeth
 Inorganic phosphate is constituent of hydroxyapatite in bone
 It provides structural support
 The formation and utilization of high energy phosphate
compounds like ATP, ADP, GTP, Creatine phosphate, etc.
contains phosphorous
 Essential for the formation of phospholipids, phosphoproteins,
nucleic acids, nucleotides (NAD, NADP, cAMP, c-GMP)

7.  Phosphate present in nucleotides, some of which function
as coenzymes, PLP, TPP, NADP and flavin coenzymes
 Several enzymes and proteins are activated by
phosphorylation (Phosphorylation & dephosphorylation)
 Mixture of HPO4
-- and H2PO4
- constitutes the phosphate
buffer which plays a role in maintaining the pH of body
fluid
 Formation of phosphate esters, such as glucose-6-p

8. Absorption and regulation
 About 90% of dietary phosphorous is absorbed
 Phosphorous is absorption from small intestine
 The absorption is stimulated by both PTH and calcitriol
 The Ca: P ratio in diet affects the absorption & excretion of
phosphorous
 Regulation of Ca & P is under the similar control mechanisms
by kidney with respect to PTH and calcitriol

9.  PTH increases calcium & phosphate release from the bone &
decreases loss of calcium & increases loss of phosphate in urine
 Excretion:
 500 mg of phosphate is excreted through urine per day
 Phosphate excretion is influenced by many factors including
muscle mass, renal function & age
 Renal threshold for phosphorous is 2 mg/dl
 Phosphates are mainly excreted by kidneys as NaH2PO4 through
the urine

10.  About 90% of the phosphate filtered at the glomeruli is
reabsorbed by the tubules
 PTH decreases the reabsorption of phosphorous from the
proximal as well as distal convoluted tubules & cause increased
excretion of phosphorous in urine
 Only small amounts are excreted in faeces
 Normal range:
 Plasma phosphorous is 2.5 to 4.5 mg/dl in adults
 In children’s it is about 5.0 to 6.0 mg/dl

11.  Calcium & phosphorous have reciprocal relationship
 In particular, if phosphate rises, calcium falls
 Fasting phosphate levels are higher
 The postprandial decrease of phosphorous is due to the
utilization of phosphorous for metabolism
 RBCs & WBCs contain a lot of phosphates
 Hemolysis should be prevented when blood is taken for
phosphate estimation

12. Hypophosphataemia
 Serum inorganic phosphate concentration <2.5 mg/dl is called as
Hypophosphataemia
 Commonly seen in conditions like:
 Hyperparathyroidism:
 High PTH increases phosphate excretion by the kidney & this
leads to low serum concentration of phosphate
 In the treatment of Diabetes the effect of insulin in causing the
shift of glucose into cells also enhances the transport of
phosphate into cells, which may result into hypophosphataemia

13.  Renal rickets is associates with low phosphate & increased
ALP concentration
 Congenital defect of tubular phosphate reabsorption, e.g.
Fanconi’s syndrome, in which phosphate is lost
 Symptoms:
 Cellular function is impaired
 Muscle pain, weakness with respiratory failure and
decreased myocardial output
 Rickets in children’s & osteomalacia in adults may develop

14. Hyperphosphataemia
 Increase in serum inorganic phosphate levels than the
normal levels is called as hyerphosphataemia
 Seen in conditions like:
 Renal failure:
 In renal failure, excretion of phosphorous is impaired,
leads to increased serum phosphate levels

15.  Hypothyroidism:
 Low PTH decreases phosphate excretion by the kidney
and leads to high serum concentration
 Symptoms:
 Increased serum phosphate levels causes decrease in
serum calcium concentration; therefore tetany &
seizures may be the presenting symptoms

16. Thank You