Amplification of P450c21 expression in cultured mammalian cells

Bone marrow cell trafficking following intravenous administration

To address trafficking of transplanted marrow cells immediately after intravenous infusion, we examined the early fate of infused non-adherent, low-density donor bone marrow cells in a syngeneic mouse model. The presence of infused donor cells, marked with indium-111 oxine (111In), with the fluorescent dye PKH26, or by a detectable transgene marker, was evaluated at 3-48 h in a variety of tissues, including peripheral blood. All three cell-marking methods indicated a rapid (< 4 h) influx of cells into the bone marrow, liver, spleen, muscle and other tissues. Moreover, these tissues remained positive for the 48 h observation period. Interestingly, analysis of PKH26-positive cells in non-myeloablated animals demonstrated that approximately 17% of infused donor marrow cells localized to the marrow space within 15 h, whereas a smaller proportion of donor cells (approximately 1-2%) localized to the marrow in recipients preconditioned by irradiation. In an effort to enrich for cells that specifically home to the bone marrow, PKH26-labelled donor marrow cells were recovered from the first host and infused into a secondary recipient. Although this was a phenotypically undefined population of cells, no increase was observed in the relative fraction of PKH26-labelled cells returning or 'homing' to the marrow of the second recipient. Taken together, these data suggest both that marrow engraftment may be mediated by non-specific 'seeding' rather than a specific homing signal, and that efficient targeting of transplanted cells to the marrow is a complex multifaceted process.

Characterization of the consequence of a novel Glu-380 to Asp mutation by expression of functional P450c21 in Escherichia coli

P450c21 catalyzes an important step in steroid synthesis. Its deficiency leads to symptoms of steroid imbalance. To obtain enough P450c21 for structure and function studies, we developed a method to express P450c21 in Escherichia coli. The 5'-region of the human P450c21 cDNA was modified to ensure efficient translation and the C terminus of the protein was extended with four His residues for easy purification. Mutant proteins with substitutions at residues 172 and 281 exhibited decreased enzymatic activities similar to those found in mammalian cells. One new mutation changing Glu-380 to Asp (D380) caused 3-fold reduction in enzymatic activity. The amount of apoprotein production detected by immunoblotting and the affinity of the mutant protein towards substrate as measured by Km were normal. The defect lies in the decreased ability of the apoprotein to bind heme, which was measured by CO difference and substrate-binding spectra. The D380 mutant protein had 3-fold reduction in peak heights in both spectra. This reduced heme binding resulted in 3-fold lower enzymatic activity.

The common I172N mutation causes conformational change of cytochrome P450c21 revealed by systematic mutation, kinetic, and structural studies

We have investigated the structure and function of P450c21 with regard to a conserved site around Ile-172 by site-directed mutagenesis making single amino acid substitutions of residues 169 173. Substitutions of Ile-171 and -172 resulted in production of mutant proteins with dramatic reductions in enzymatic activities, indicating the importance of these two residues in maintaining the structure and function of P450c21. The I171N protein was present at a slightly lower level, due to a decreased rate of protein synthesis. The I172N apoprotein was synthesized at the normal rate, but its heme-bound P450 form was present at a much lower level. This I172N protein was tightly integrated into the membrane of endoplasmic reticulum, similar to the wild type P450c21, as shown by immunofluorescence detection, alkaline extraction, and cellular fractionation. Kinetic studies indicated that I172N had a lower Vmax value. In addition, the I172N protein was more sensitive to proteinase K digestion, indicating a possible alteration of conformation. This conformational change may result in the lower yield of the I172N hemoprotein and the reduced catalytic activity.