Table 1.
Substrate specificity of Cyp7b
| Substrate | % conversion*
|
Km†; Vmax‡ | |
|---|---|---|---|
| Major product | Minor product(s) | ||
| DHEA | 95 | 2; 1§ | 13.6; 303‖ |
| Pregnenolone | 70 | 3; <1 | 4.0; 35.9‖ |
| 25-Hydroxycholesterol | 30 | <0.1¶ | ND |
| 5α,Androstane-3β,17β-diol | 11 | 9.1 | ND |
| 17β-Estradiol | 5 | 1 | 7.5; 2.4** |
| Testosterone | 1.2 | 0.4; 0.3; 0.3; 0.2 | ND |
| Progesterone | 0.6 | 0.2 | ND |
| Corticosterone | <0.5 | ||
| Cortisol | <0.5 | ||
| Androstenedione | <0.5 | ||
| Dihydrotestosterone | <0.5 | ||
ND, not determined.
*
Values are expressed as the ratio of product radioactivity to total substrate radioactivity in a standard reaction (200 μl, 20 min, 37°C) utilizing 1 nmol substrate (means of two determinations).
†
The apparent Km is expressed in μM and was measured for 7-hydroxylation alone.
‡
The apparent Vmax is expressed as pmol/min/mg total protein; because the concentration of Cyp7b enzyme in the extract is not known these data are purely comparative.
§
Product comigrating with 7β-hydroxy DHEA on TLC.
¶
Products including cholesten-5-ene,2,3β,7α,25-tetrol (not shown).
‖
Means of three determinations.
**
Means of two determinations.