Biotin recycling impairment in phenylketonuric children with seborrheic dermatitis

Objective To investigate the effect of a therapeutic diet on serum biotin l evels and to explain the seborrheic dermatitis in phenylketonuric (PKU) pat ients on a "loose" diet. Design Forty-seven patients were divided into two groups: group A (n = 21) demonstrated good compliance to a special diet and group B (n = 26) were on a "loose" diet. Most of the patients in group B (20/26), who suffered from mild seborrheic dermatitis, were requested to return to phenylalanine (Phe )-restricted diet for at least 15 days. Seventy-nine healthy children of co mparable age were used as controls. Biotin serum levels and plasma biotinid ase activity were measured in patients as well as controls. In addition, bi otinidase activity was evaluated in vitro after incubation with various con centrations of Phe. Results Biotin levels in group A patients (636 +/- 118 ng/L) were statistic ally significantly elevated (P < 0.01) compared with those of group B patie nts before (412 +/- 184 ng/L) and after(501 +/- 160 ng/L) 15 days on a Phe- restricted diet, as well as with those of controls (337 +/- 290 ng/L). Furt hermore, biotinidase activities were decreased in group B patients (4.2 +/- 1.68 nmol/min/L) compared with those of group A patients (6.4 +/- 0.7 nmol /min/L) and controls (6.10 +/- 0.8 nmol/min/L). Additionally, biotinidase a ctivities in the patients of group B were restored to normal (5.78 +/- 0.81 nmol/min/L), with a simultaneous remission of their skin lesions, after 15 days on a Phe-restricted diet. Moreover, the in vitro findings showed a 51 % inhibition of biotinidase activity when incubated with Phe (20 mg/dL). Conclusions It is suggested that the high biotin levels in group A patients reflect the intake of water-soluble biotin of vegetable origin. In contras t, the low biotinidase activity in group B patients may be attributed to th eir high Phe plasma levels, which acts as an enzyme inhibitor, as shown by the in vivo and in vitro results. Consequently, the observed seborrheic der matitis in PKU children (group B) is associated with an impairment of bioti n recycling.