Henning, 47, was part of an aid convoy taking medical supplies to a hospital in northwest Syria in December last year when it was stopped by gunmen and he was abducted.
He appeared in a video released by IS last week, which showed the murder of another Briton, David Haines. In it, a masked man said Henning would also be killed if British Prime Minister David Cameron kept supporting the fight against IS.
"Alan is a peaceful, selfless man," his wife Barbara said in a statement released via Britain's Foreign Office late on Saturday.
"When he was taken he was driving an ambulance full of food and water to be handed out to anyone in need. His purpose for being there was no more and no less."
Last week, Muslim groups across Britain, including some organizations that are highly critical of British foreign policy and blame Western interference for fanning the recent crisis in Iraq and Syria, called for Henning's release.
"I cannot see how it could assist any state's cause to allow the world to see a man like Alan dying," his wife said, saying she had tried to contact his captors but received no response.
"I pray that the people holding Alan respond to my messages and contact me before it is too late ... I implore the people of the Islamic State to see it in their hearts to release my husband." REUTERS