What is HCU :

People with Homocystinuria (HCU)  cannot digest the amino acid called Methionine. HCU,  is similar to  Phenylketonuria (PKU),  Methylmalonic Acidemia(MMA)  and Tyrosinemia. Each of these disorders require food products which are low in particular types of amino-acids. i.e. Methonine in the case of HCU, Phenylalanine in the case of PKU.

Detailed Explanation:

Homocystinuria is an inborn error of methonine metabolism and was discovered in 1962 independently in Northern Ireland by Carson and Neill and in the United States by Gerritsen et al. The basic defect was defined by Mudd and his co-workers in 1964 as a deficiency of the hepatic enzyme Cystathionine ▀ Synthase. ( Nina A.J. Carson, The Treatment of Inherited Metabolic Disorders)

A simple explanation of the disorders such as  HCU  is given below. Methonine, an amino acid, which is present in regular food protein undergoes the following steps in digestion in the body







                     »Cystathionine synthase

                    (absent in Homocystinuria)




The conversion of Methonine to Homocyst(e)ine takes place without any problem but Homocyst(e)ine to Cyst(e)ine does not happen because of absence of the enzyme, Cystathionine b-synthase. Homocyst(e)ine  is not converted to form Cyst(e)ine. This leads to accumulation of  Homocyst(e)ine, which is toxic, and  deficiency of Cyst(e)ine, which is an essential amino acid in Homocystinuria patients. ( In healthy individual, Cyst(e)ine is made by the body but since HCU patients do not make this amino acid in their body it needs to be supplemented). In order to reduce the toxicity of accumulated Homocyst(e)ine, only low-protein diet products which are low in  Methonine are given regularly. The protein required to maintain proper growth, is given in the form of supplement which is free from  Methonine.


What do you do:


Since protein in the food contains all amino acids including Methonine use mainly low-protein food for your regular meals. Since you need protein for the body all other amino acids except methonine is taken additionally. This is very important and it is not a supplement such as a vitamin. It is necessary for your survival and it is also called as 'formula'.

What happens if you do not follow the diet:

This leads to accumulation of  Homocyst(e)ine, which is toxic. This can lead to stroke, eye lens dislocation, mental retardation, seizures and many other problems.