PKU, Sorbitol, & Galactose/Fructose Disorders

Phenylketonuria (PKU)

Phenylketonuria (PKU) is a genetic deficiency of either Phenylalanine Hydroxylase or its cofactor, Tetrahydrobiopterin (THB). This inhibits the conversion of Phenylalanine to Tyrosine. The lack of Tyrosine causes multiple health outcomes and the excess Phenylalanine is excreted in the urine as it builds up in the body. Untreated PKU is characterized by Mental Retardation and Seizures. A mild form of Albinism results from less tyrosine being available for melanin synthesis. The build-up of metabolites with aromatic properties leads to a Musty/Mousy Body Odor.

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Tyrosine is a non-essential amino acid as most people can create tyrosine by breaking down phenyalanine. However, tyrosine becomes essential in PKU. PKU is treated by supplementing tyrosine and restricting phenyalanine intake in the diet. Items that contain Aspartame (like diet soda) need to be avoided as well as aspartame is broken down into phenylalanine.


In tissues, some glucose is converted to an alcohol called Sorbitol. Small amounts of sorbitol allow glucose to be sequestered in the tissue and do not cause a problem. Sorbitol Dehydrogenase converts sorbitol to harmless fructose to prevent sorbitol build up. However, some tissues (primarily in the eye) do not have enough Sorbitol Dehydrogenase to handle excessive amounts of sorbitol. Therefore, patients with chronic hyperglycemia (Diabetic Patients) have a buildup of sorbitol which can cause Cataracts and other damage to the visual pathway.

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Fructose & Galactose Disorders

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Essential Fructosuria is an asymptomatic deficiency of Fructokinase. Hereditary Fructose Intolerance is a deficiency of Aldolase B that causes hypoglycemia, hepatomegaly, jaundice & vomiting. These patients should avoid fructose & sucrose (which contains fructose).

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Galactokinase Deficiency causes cataracts. Galactosemia is a deficiency of Galacto-1-P Uridyl Transferase that leads to cataracts, hepatomegaly, jaundice & retardation. Patients with both disorders should avoid lactose as it contains galactose.

All four disorders above are Autosomal Recessive inheritance. Whichever sugar builds up, either the sugar by itself or the phosphorylated sugar, can be detected in excess in the urine and blood. Galactose disorders are more serious than fructose disorders because excess fructose is just excreted in urine while excess Galactose can be converted to Galactitol (alcohol) which can accumulate in tissues. Fructose disorders don’t appear until the baby stops breastfeeding and is exposed to formula or other dietary sources with fructose. Lactose is a disaccharide of 1 Glucose and 1 Galactose. Therefore must be avoided in Galactose Disorders.  Sucrose is a disaccharide of 1 Glucose and 1 Fructose. Therefore, Sucrose (table sugar) must be avoided in Fructose Intolerance.


Now that you have finished this video you should check out the next video in the Biochemistry sections which covers Alcohol Metabolism, Alcoholism & Methanol Poisoning.  



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7 thoughts on “PKU, Sorbitol, & Galactose/Fructose Disorders”

  1. Hi Brian
    My name is Les Joseph, I’m an international med student. I stumbled over your videos today on youtube, let me tell you it was a blessing. In the near future, I will be donating to help you further this great work. My friend, you have a gift, Again, I want to thank you. May God keep you healthy so you can continue.

    1. Wow! Thank you so much for the kind words! Hearing from my viewers makes all of the hard work worth it. I’m working on the next few videos now so keep an eye out for those. Good luck with the studying

  2. Hi friend would you include mode of heritance of these enzyme deficiencies. As I remember enzyme deficiencies are usually are autosomal recessive or X-linked recessive disorders. great effort

  3. Nice effort. one day, Stomp on step1 will be the first reference for USMLE step1 exam. Your hard work will pay off.

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