The uptake of
all-trans-retinoic acid (RA) and two new
retinoids [4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbamoyl )
benzoic acid (
Am80) and (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]
benzoic acid (
Ch55)] by HL-60 human promyelocytic
leukemia cells was investigated. For the investigation, [3H]RA and [3H]
Am80 with high specific radioactivities (more than 50 Ci/mmol) were used. [3H]
Am80 was prepared by hydrogenolysis of the corresponding chlorinated derivative of
Am80 with
tritium gas. The
retinoids RA,
Am80 and
Ch55 were efficiently taken up by HL-60 cells, and induced differentiation of the cells into mature granulocytes. The specific bindings (uptake) of RA,
Am80 and
Ch55 (the bindings inhibited competitively by the other two
retinoids) by HL-60 cells were due to a newly detected
binding protein. The
protein that bound specifically to RA appeared identical to that which bound specifically to
Am80 by high-performance liquid chromatography (HPLC), and was named
retinoid-specific
binding protein (RSBP). One HL-60 cell was found to contain about 1500 molecules of RSBP distributed between the nuclear fraction and cytosolic fraction in proportions of about 4:1. The bindings of the three
retinoids (RA,
Am80 and
Ch55) to RSBP (i.e., formation of
retinoid-RSBP complexes) greatly enhanced the affinity of RSBP for the nuclei. The apparent molecular weight of RSBP was estimated to be 95,000 daltons by size exclusion HPLC. The association constants (Ka) of RSBP were calculated to be 2.4 X 10(10) M-1 for RA and 4.4 X 10(10) M-1 for
Am80 from Scatchard plots. The bindings of RA,
Am80 and
Ch55 to RSBP were mutually competitive, indicating that the binding sites for RA,
Am80 and
Ch55 were identical. The very high affinities of these
retinoids for RSBP (Ka's of the order of 10(10) M-1) correspond to the effective concentrations of these
retinoids in HL-60 cell culture medium for induction of differentiation of the cells. The mutually competitive bindings of these
retinoids strongly support the idea that RSBP is the true receptor of
retinoids.