The reductase activity in patients with

The 5α-reductase activity in patients with 21-OHD, as assessed by the 5α-THF to 5β-THF (ratio 3) and by the 5α-17HP to 17HP (ratio 4) ratios, showed a similar pattern of activities (Fig. 2A–C) and significant correlations (ratio 1 vs. ratio 3; rs=0.67; p<0.0001; ratio 1vs. ratio 4: rs=0.56; p<0.0001) when compared to the relative backdoor pathway activity until the generation of 5α-17HP (ratio 1). Both 5α- to 5β-ratios showed a significant correlation (ratio 3vs. ratio 4: rs=0.62; p<0.0001), thus demonstrating that both ratios accurately describe 5α-reductase activity in patients with 21-OHD despite the altered steroid metabolism. In Papain Inhibitor to the 5α-reducase activity, the activity of the backdoor pathway until the generation of An, which is in contrast to 5α-17HP located behind the 17,20-lyase, showed a different time course with highest activity in the newborn and infant groups (Fig. 2D). This demonstrates that the An generation is modulated by the 17,20-lyase activity of CYP17A1. Patients with 21-OHD showed elevated urinary concentrations of metabolites of 17-OHP leading to elevated 17-hydroxy C21 steroid concentrations when compared to controls (p<0.0001 for all age groups; Fig. 3A). Additionally, the concentration of the C19 steroids 11OH-An and Et were higher in patients with 21-OHD (p<0.0001 for all age groups; Fig. 3B). The 17,20-lyase activity was highest in the oldest age group in both, controls and patients with 21-OHD. The 17,20-lyase activity was low during neonatal age and infancy in controls, but relatively high in neonates and infants with 21-OHD (Fig. 3C). A comparison of the median values of this ratio revealed that in the control group, the median values of the neonatal and infant groups reached only 1/36th and 1/20th of the median value of the group >9.0years. In contrast, in the 21-OHD group, the median values reached 1/4th in both the neonatal and the infant groups compared with the values of the group >9.0years. The 17,20-lyase activity of the backdoor pathway in patients with 21-OHD was higher in the newborn group than in the infant group. After infancy the activity rose again with increasing age (Fig. 3D). Table 4 and Fig. 4A–C provide an overview of the different pathway activities in the different age groups in patients with 21-OHD.
Discussion The 5α-reductase and the 17,20-lyase activity of CYP17A1 compete about 17-OHP as their substrate. Therefore, the direction of the conversion of 17-OHP into the classical Δ4 and Δ5 or backdoor pathways depends on their relative activities and substrate affinities. We could show a similar pattern of activities and significant correlations between the 5α-reductase and the backdoor pathway until the generation of 5α-17HP. This suggests that 5α-reductase determines the initial activity of the alternative backdoor route and promotes the generation of 5α-17HP in cases of 17-OHP accumulation as in 21-OHD. Our data demonstrate the crucial role of 5α-reductase as the gatekeeper to the alternative backdoor pathway. It has been shown that the type 1 5α-reductase isoform (encoded by SRD5A1) is responsible for androstanediol production in immature mouse testes in the backdoor route [17]. However, the 5α-reductase activity in patients with 21-OHD increased after the first month of life and fell thereafter, concordant with the expression of the type 2 isoform (encoded by SRD5A2) of 5α-reductase [18]. We have previously demonstrated a higher increase of the Δ4A generation compared with that of 5α-17HP in newborns with 21-OHD [4]. Our additional analysis of 5α-reductase and 17,20-lyase activities clearly shows that this is due to relatively low 5α-reductase activity and high 17,20-lyase activity in the neonate, thus promoting the classical pathway. However, the latter is additionally responsible for the conversion of 5α-17HP to An. As a consequence, the elevated An to Et ratio in neonates indicates a better flux through the backdoor pathway compared to the flux through the classical pathway, as demonstrated by high 17,20-lyase activity in the backdoor pathway in this age group. Thus, the 17,20-lyase activity modulates the flux through the backdoor pathway. Our calculation of the 17,20-lyase activity of the backdoor pathway, i.e. of the conversion of 5α-17HP to An, was based on the known Et concentrations, and the correlation between An and Et in controls was used to assess the amount of An derived from the backdoor pathway in patients with 21-OHD. As our equation is independent from age, this requires that the An to Et ratio is irrespective of age, as it was demonstrated previously in our laboratory in a large series of 400 healthy children aged three to 18years [11]. An increasing 5α-reductase activity in parallel with a decreasing 17,20-lyase activity leads to a more efficient 5α-reduction of 17OHP to 5α-17HP compared to its conversion to An during infancy [4]. The decreasing 17,20-lyase activity leads to a lower flux through the backdoor pathway and to a decreasing generation of An, despite high 5α-17HP generation in this age group. However, a low 17,20-lyase activity would led to accumulation of 5α-17HP in the presence of substantial 5α-reductase activity. As this would be interpreted as an increased flux via the backdoor pathway, the activities of 5α-reductase and 17,20-lyase interact with each other in our model. During childhood and adolescence, the 17,20-lyase activity of the classical pathways increased, whereas a decreasing 5α-reductase activity leads to a diminished role of the backdoor pathway for androgenic steroid production.