N-acetyl-alpha-neuraminosyl-(2->8)-N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-beta-D-galactosaminyl-(1->4)-[N-acetyl-alpha-neuraminosyl-(2->8)-N-acetyl-alpha-neuraminosyl-(2->3)]-beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1')-N-(octadecanoyl)-sphing-4-enine + H2O = ganglioside GT1b (d18:1(4E)/18:0) + N-acetylneuraminate — reaction