Nitrogen Excretion And Urea Cycle

Introduction

The degradation of dietary and body protein yield amino acids. they are deaminated early in catabolism and the resulting nitrogen and carbon skeleton are processed independently. The a-amino group of most amino acids is removed by amino transferases and for other amino acids by specific enzyme such histidine and serine /threonine dehydratase. The ammonia production from extra- hepatic tissue such as muscles and other tissue (except muscle ) is transported to the liver in the form of alanine and glutamine ,respectively. free ammonia is toxin so it is detoxified to urea before excretion, in ureotelic organisms.

  

In this unit we shall learn about nitrogen balance ,transamination reaction ammonia production ,Glc -Ala cycle ,urea cycle and Krebs bicycle. Lastly we will discuss the disorders association with deficiencies in enzyme  of the urea cycle.

Nitrogen Balance

protein constitute the major nitrogen containing macromolecules that perform wide range of roles .the intake nitrogen is primarily in the difference between the amount of nitrogen ingested and nitrogen excreted in feces ,urine (primarily  as urea ) and through perspiration. an individual may be in zero (n equilibrium), positive or negative nitrogen balance.

Zero nitrogen balance(N- equilibrium)

when nitrogen intake matches nitrogen output it is known as zero nitrogen balance .in other words protein synthesis is equal to protein degradation this type of situation is typically found in adults. zero nitrogen balance may be represented as:

nitrogen intake= nitrogen output

positive nitrogen balance

when intake of nitrogen is greater than the excretion of nitrogen ,it is known as positive nitrogen balance. this type of situation is observed during pregnancy childhood and adolescence. positive nitrogen balance is given as:

nitrogen intake>nitrogen excretion

negative nitrogen balance

when nitrogen output exceed intake ,it result in negative nitrogen balance in other words protein synthesis is less than protein degradation.it is generally encountered in people who ingest inadequate amount or low quality protein ;post surgical patients; infection and thosein advanced stage of cancer .it is important to mention here that a deficiency of even one amino acid results in negative nitrogen balance (all or none law ) the quality of a protein is directly related to its biological value (%of absorbed nitrogen retained by the body) prolonged periods of negative nitrogen balance  can be fatal if the loss of body protein reaches about one -third of the total protein. negative nitrogen balance can be represented as: 

nitrogen intake <nitrogen excretion

we shall now briefly elaborate why quality of proteins differs from one source to another. the quality of a protein is dependently on the essential amino acid content .some proteins either have one or more essential amino acids in limited amounts or do not supply all essential amino acids such proteins are classified as incomplete and are generally derived from plant sources therefore vegetarians are advised to consume a combination diet ,for instance of cereals and pulses to satisfy their daily requirement of essential amino acids animal protein (milk ,eggs )on the other hand are complete proteins.