Adrenarche is the term for the maturational increase in adrenal androgen production that becomes biochemically apparent as a rise in dehydroepiandrosterone sulfate (DHEAS) at about six years of age in both girls and boys ( figure 1 ). It represents a gradual change in the pattern of adrenal secretory response to corticotropin (adrenocorticotropic hormone, ACTH), characterized by a disproportionate rise of 17-hydroxypregnenolone and dehydroepiandrosterone (DHEA) relative to cortisol. This change primarily occurs in the zona reticularis of the adrenal cortex, the cells of which express a unique pattern of steroidogenic enzymes that includes expression of sulfotransferase 2A1 (SULT2A1), which catalyzes the formation of DHEAS, the major adrenarchal steroid ( figure 2 ). (See "Normal adrenarche" .)
Biosynthesis of steroid hormones requires a battery of oxidative enzymes located in both mitochondria and endoplasmic reticulum. The rate-limiting step in this process is the transport of free cholesterol from the cytoplasm into mitochondria. Within mitochondria, cholesterol is converted to pregnenolone by an enzyme in the inner membrane called CYP11A1. Pregnenolone itself is not a hormone, but is the immediate precursor for the synthesis of all of the steroid hormones. The following table delineates the enzymes required to synthesize the major classes of steroid hormones.
Stop codon suppression or translational readthrough occurs when in translation a stop codon is interpreted as a sense codon, that is, when a (standard) amino acid is 'encoded' by the stop codon. Mutated tRNAs can be the cause of readthrough, but also certain nucleotide motifs close to the stop codon. Translational readthrough is very common in viruses and bacteria, and has also been found as a gene regulatory principle in humans, yeasts, bacteria and drosophila.   This kind of endogenous translational readthough constitutes a variation of the genetic code , because a stop codon codes for an amino acid. In the case of human malate dehydrogenase , the stop codon is read through with a frequency of about 4%.  Amino acids inserted at the stop codon depends of the identity of the stop codon itself: Gln, Tyr and Lys; have been found for UAA and UAG codon, while Cys, Trp, Arg for UGA codon have been identified by mass spectrometry.