Problem Barry B

Barry is the second child of unrelated parents; his brother is 5 years old, fit and healthy. Barry was born at full term after an uneventful pregnancy, weighing 3.4 kg (the 50th centile), and developed normally until he was 6 months old, after which time he showed some retardation of development. He also developed a fine scaly skin rash about this time, and his hair, which had been normal, became thin and sparse.

At 9 months of age he was admitted to UCL Hospital in a coma; the results of clinical chemistry tests on a plasma sample are shown in Table 10.5.

The acidosis was treated by intravenous infusion of bicarbonate, and he recovered consciousness. Over the next few days he continued to show signs of acidosis (rapid respiration), and even after a meal he showed ketonuria. His plasma lactate, pyruvate and ketones remained high; plasma glucose was in the low normal range and his plasma insulin was normal both in the fasting state and after an oral glucose load.

Urine analysis revealed the presence of significant amounts of a number of organic acids that are not normally excreted in the urine, including:

  • lactate, pyruvate and alanine;
  • propionate, hydroxypropionate and propionylglycine;
  • methyl citrate;
  • tiglate and tiglylglycine;
  • 3-methylcrotonate, 3-methylcrotonylglycine and 3-hydroxyisovalerate.

His skin rash and hair loss were reminiscent of the signs of biotin deficiency, as caused by excessive consumption of uncooked egg-white (section 11.12). However, his mother said that Barry did not eat raw or undercooked eggs at all, although he was fond of hard-boiled eggs and yeast extract (which are rich sources of biotin). His plasma biotin was 0.2 nmol/L (normal level > 0.8 nmol/L), and he excreted a

Table 10.5 Clinical chemistry results for a plasma sample from Barry B on admission and reference range for 24 hours fasting

Barry B

Reference range

Glucose (mmol/L)

3.3

3.5-5.5

PH

6.9

7.35-7.45

Bicarbonate (mmol/L)

2.0

21-25

Ketone bodies (mmol/L)

21

1-2.5

Lactate (mmol/L)

7.3

0.5-2.2

Pyruvate (mmol/L)

0.31

<0.15

significant amount of biotin in the form of biocytin and small biocytin-containing peptides (see Figure 11.25); these are not normally detectable in urine.

He was treated with 5 mg of biotin per day. After 3 days the various abnormal organic acids were no longer detectable in his urine, and his plasma lactate, pyruvate and ketones had returned to normal, although his excretion of biocytin and biocytin-containing peptides increased. At this stage he was discharged from hospital, with a supply of biotin tablets. After 3 weeks his skin rash began to clear and his hair loss ceased.

Three months later, at a regular out-patient visit, it was decided to cease the biotin supplements. Within a week the abnormal organic acids were detected in his urine again and he was treated with varying doses of biotin until the organicaciduria ceased. This was achieved at an intake of 150 ^g/day (compared with the reference intake of 10—20 ^g/day for an infant under 2 years old).

He has continued to take 150 ^g of biotin daily, and has remained in good health for the last 4 years.

Can you account for the biochemical basis of Barry's problem?

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