Differential effects of biotin deficiency and replenishment on rat liver pyruvate and propionyl-CoA carboxylases and on their mRNAs.

Although the role of vitamins as prosthetic groups of enzymes is well known, their participation in the regulation of their genetic expression has been much less explored. We studied the effect of biotin on the genetic expression of rat liver mitochondrial carboxylases: pyruvate carboxylase (PC), propionyl-CoA carboxylase (PCC), and 3-methylcrotonyl-CoA carboxylase (MCC). Rats were made biotin-deficient and were sacrificed after 8 to 10 weeks, when deficiency manifestations began to appear. At this time, hepatic PCC activity was 20% of the control values or lower, and there was an abnormally high urinary excretion of 3-hydroxyisovaleric acid, a marker of biotin deficiency. Biotin was added to deficient primary cultured hepatocytes. It took at least 24 h after the addition of biotin for PCC to achieve control activity and biotinylation levels, whereas PC became active and fully biotinylated in the first hour. The enzyme's mass was assessed in liver homogenates from biotin-deficient rats and incubated with biotin to convert the apocarboxylases into holocarboylases, which were detected by streptavidin blots. The amount of PC was minimally affected by biotin deficiency, whereas that of the alpha subunits of PCC and of MCC decreased substantially in deficient livers, which likely explains the reactivation and rebiotinylation results. The expression of PC and alphaPCC was studied at the mRNA level by Northern blots and RT/PCR; no significant changes were observed in the deficient livers. These results suggest that biotin regulates the expression of the catabolic carboxylases (PCC and MCC), that this regulation occurs after the posttranscriptional level, and that pyruvate carboxylase, a key enzyme for gluconeogenesis, Krebs cycle anaplerosis, and fatty acid synthesis, is spared of this control.

Apparent higher frequency of phenylketonuria in the Mexican state of Jalisco.

The geographic origin of Mexican patients with phenylketonuria (PKU) in Mexico City and in southern California was studied. Compared to patients with other metabolic disorders, patients with PKU were significantly more likely to have originated from the Los Altos region of the state of Jalisco and its environs. The incidence of PKU among mentally retarded students attending special education schools was found to be significantly higher in Jalisco (particularly the Los Altos region) than in the neighboring state of Guanajuato (1.09% vs 0.3%). These results strongly suggest a "population of origin" effect, the mutant allele(s) having been introduced by the Spanish ancestors of the current population. Our findings also support the addition of PKU to the neonatal screening program for this region of Mexico.