Evidence that the human placental 5-monodeiodinase is a phospholipid-requiring enzyme.

Gel filtration (GFI) of the solubilized human placental microsomes (SHPMP) performed in an Ultrogel AcA-34 column in the presence of 1 mM 3-(3-cholamidopropyl)dimethylammonio-1-propane sulfonate (CHAPS) plus 10 mM n-octyl-beta-D-glucopyranoside (beta-OG) demonstrated two main protein peaks. The 5-Monodeiodinase (5-MD) activity measured by the conversion of [125I]T3 to [125I]3,3'-diiodothyronine in a 2- to 18-h incubation at 37 C in the presence of 10 mM dithiothreitol was detected only in the first peak, and the specific activity was increased about 9 times over that of the starting SHPM. The fractions containing most of the 5-MD activity were filtered through a second Ultrogel AcA34 column (GFII) in the presence of 2 mM CHAPS plus 20 mM beta-OG. In these conditions, 5-MD activity was detected in low amounts only in the second peak. Cation exchange chromatography on carboxymethylcellulose-Sephadex with a starting buffer of pH 5 containing 2 mM CHAPS plus 20 mM beta-OG, followed by a pH 8 buffer, showed a very small OD peak at the void volume (P) and a second peak with about 95% of the protein (E). However, no 5-MD activity was detectable in either peak, while a nearly complete restoration of the enzyme was achieved when P and E were mixed. 5-MD was also completely restored by combination of P with the first inactive peak of GFII. When P was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, no distinct protein bands were observed. After ethanol-ether extraction and digestion with H2SO4 and H2O2, inorganic phosphate was detectable only in P, suggesting the presence of phospholipids. We next studied the effect of phosphatidyl serine (PS), phosphatidyl choline (PC), or phosphatidyl ethanolamine (PE) on 5-MD activity of E (5 micrograms protein/mL). The 5-MD activity was recovered in a dose-response manner with all phospholipids studied, but PS was the most effective agent for reconstitution. At 1 microgram/mL, 5-MD activity, expressed as a percentage of the total P plus E activity, was 101% for PS, 35% for PC, and 20% for PE. The addition of rat liver or kidney microsomes (80 micrograms/mL) to E (5 micrograms/mL) provided recoveries of 79% and 48%, respectively, of the total P plus E activity. The following conclusions were reached. 1) Phospholipids are essential for the 5-MD activity of SHPMP. 2) CHAPS and beta-OG may extract phospholipids from the membranes without denaturation of the 5-MD.(ABSTRACT TRUNCATED AT 400 WORDS)