The levels of neopterin, biopterin and the neopterin/biopterin ratio (NIB) were measured in urine samples taken from normal young and elderly control subjects, exceptionally healthy elderly control subjects classified according to the 'Senieur' protocol and patients with Down`s syndrome (DS ) or Alzheimer's disease (AD). The NIB ratio was approximately unity in the control groups with the exception of the normal elderly controls. The levels of neopterin and biopterin declined with age in the exceptionally healthy 'Senieur' control group. The NIB ratio was elevated in young and old DS patients as a result of a significant increase in neopterin. Neopterin levels were significantly elevated in AD patients compared with the healthy elderly controls, but this did not result in a significant increase in the NIB ratio in these patients. The NIB ratio increased with age in AD patients as a result of a decline in biopterin. These results suggested that there is a cellular immune response in DS and AD patients which in DS, may precede the formation of β-amyloid deposits in the brain. In addition, there may be a deficiency in tetrahydrobiopterin biosynthesis in AD which becomes more marked with age.
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